Does the Catholic Church teach that some who died before Christ are still in Limbo? Why?

We must distinguish between the two limbos:

1. Limbo of hell or of the Patriarchs (limbus inferni seu patrum)
   This is also known as Abraham's bosom. This limbo no longer exists (cf. "Reply to Objection 3" of this) because Christ has already descended into hell and brought those souls detained there to heaven during His Ascension.
2. Limbo for children (limbus puerorum)
   This is different than the limbo of the Fathers, which "is placed higher than the limbo of children." (cf. "I answer that…" of this). It is where the souls of unbaptized children are. It is a place of perpetual, natural happiness where souls are not actively punished by fire because they have not committed any personal sins; however, it is still hell, which, by definition, is the place where one is deprived of the Beatific Vision.

As St. Thomas Aquinas writes ("I answer that…" of this):

The limbo of the Fathers and the limbo of children, without any doubt, differ as to the quality of punishment or reward. For children have no hope of the blessed life, as the Fathers in limbo had, in whom, moreover, shone forth the light of faith and grace. But as regards their situation, there is reason to believe that the place of both is the same; except that the limbo of the Fathers is placed higher than the limbo of children, just as we have stated in reference to limbo and hell (Article [5]).

It seems Dante is confounding these two types of limbo.

It's possible but very unlikely that "the Roman poets (and perhaps others who died before Christ's death)"—i.e., those ignorant of Christ and the Catholic Faith—have never committed a sin are in a similar state as those in the limbo for children, with one difference: they had the use of reason.

The best commentary on Dante's Divina Commedia is

• Dante Alighieri, and Fr. Giovanni Maria Cornoldi. La Divina Commedia di Dante Alighieri, col comento. Rome: Befani, 1887.

The commentary on Inferno Canto IV begins on p. 45. Here's a rough translation of the relevant part; I welcome any corrections/improvements of it:

To understand correctly what Dante will here say, it is necessary to presuppose the following doctrine, which is as important as it is little-understood.

1. If Adam had not sinned, his children would have been procreated with original justice and thus with the soul of the same; at the moment of its creation and union to the body, it would be ordained with sanctifying grace and the strengths that are included in the concept of this justice. This was a free gift not due to the natural perfection of Adam or of his children; therefore, it is considered as precious garments given by God to Adam, with which garments his children would have been conceived.

2. God decreed that if Adam sinned, he would disinherit his children of this supernatural gift. He acted like a prince who raised one of his subjects to an adoptive son; he gives a fief to transmit to the offspring, but provided that he remain faithful. The moment he lacked fidelity, he would lose, not only for himself but also for his children, the fief itself. Through such a thing, through the sin of Adam, men would not lose what pertains to human nature, nor what raises it, but what of it is not absolutely required; and the Aquinate [St. Thomas Aquinas] will say this below (n. 5) in a magnificent passage. Wherefore, if one wants to understand man in a state of pure nature (in which there was never in reality, because once created he was raised to the order of grace), it would differ from a man who is conceived with the privation of the gift of original justice, as one who is naked differs from one who is undressed. In the undressed man, a negation that is also a privation is indicated—the word "privation" meaning the lack of that which one is due to have. If the supernatural and preternatural gifts, the sum of which the original justice consisted, did not have a beneficent influence also on the natural faculties of man, through such a thing, the subtraction of these gifts happens because of sin, the intellect, will, and sense faculties undergoing damage.
   
   Because, after the sin of Adam, the tradition of many revealed truths, especially with respect to moral truths, still remains, and God increased his graces, it seems that if one can affirm that we do not have sufficient reason to say that humankind in the state of pure nature—i.e., if he was not raised to the supernatural order—would have been better because of this than it was in general for all the world before the advent of Christ. It would perhaps have been a hundred times worse.
   
   Others will say: therefore human nature will be said to be intrinsically deprived. The inference does not follow; but rather it is a nature perfect in its order, but this order is imperfect, being abandoned to its own forces. Therefore, also due to its weakness it was convenient that it was raised to the supernatural and divine order, made a participant by adoption and grace of the divine perfection and nature. Likewise, gold, gems, marble, etc. left underground is perfect considered in its essence, but has a capacity to acquire by human art a perfection of much greater extent; so man, for his benefit and enjoyment, feels inclined to give them this greater perfection and in them explain, working them, his own ideas and even impress on them his own image.

3. The Angelic Doctor, St. Thomas, gives that comparison:

   …ever since the sin of the first parent, all men come into the world bereft of original justice and burdened with the defects that attend its loss. This is in no way against the order of justice, as though God were punishing the sons for the crime of their first father. For the punishment in question is no more than the withdrawing of goods that were supernaturally granted by God to the first man for transmission, through him, to others. These others had no right to such goods, except so far as the gifts were to be passed on to them through their first parent. In the same way a king may reward a soldier with the grant of an estate, which is to be handed on by him to his heirs. If the soldier then commits a crime against the king, and so is adjudged to forfeit the estate, it cannot afterwards pass to his heirs. In this case the sons are justly dispossessed in consequence of their father’s crime. (Compendium Theologiæ c. 195).

The reason of the sin is not in not having original justice, but in not having it according to the purpose God made for having it. So, children of the feudal lord are not said to be degraded for not having the fief, but for not having it according to how the father desired them to have it. This does not matter in these personal faults, but original sin is not said to be personal, but an infection that they contract because they are conceived by the parent whose sin was truly a personal fault.

4. In this first circle, Dante places them who died without grave personal fault [i.e., without mortal sin] and with original sin, because it was not given them that which, by divine will, was and is an active means for abolishing original sin and causing sanctifying grace. Here two classes of persons must be distinguished. The first contains those who, because of a lack of the use of reason, are incapable of doing, with free will, good and using actual grace. This are the children who die without baptism. Firstly, these, because the did not have any personal sin, do not suffer torments or those punishments that are called of the sense. Secondly, they will not have the beatific vision of God and the consequent supernatural happiness. Inasmuch as in this is constituted the supernatural end, which cannot be obtained without sanctifying grace, of which the soul of these is deprived by force of their original sin. Thirdly, by this privation they must be said to be damned and also disinherited; but they will not have, by this, affliction, because they, not having the use of reason, will not use it for good, and they will enjoy with the natural faculties. This soft doctrine is diametrically opposed to that which places these children alongside those other reprobates who committed personal sin and who are [justly] tormented by fire. Behold what the Angelic Doctor, St. Thomas, says:

   Every man having freewill is proportioned attaining eternal life, because he can prepare himself for grace, through which he merits eternal life: and thus if they lack this, there is maximal pain for them, because they admit that which was possible for him. Children thus were not proportioned to this, that they would have eternal life; because it is not due to them from the principles of nature, because it exceeds every natural faculty, nor to their proper acts to be able to attain any such good alone: and thus none whatsoever suffers of the loss of the divine vision; indeed, they enjoy more of that which many participate of divine good, and by natural perfections. Nor can it be said that they were proportioned to attaining eternal life, although not not by his action, but by the action of those around them, because they are brought by others to be baptized, as also many children in the same conditions are baptized, they attained eternal life; this is a superabundance of grace that some without their own action merit; thus such a defect of grace does not cause more sadness in children dying unbaptized, than in wise men who had no grace, or in others like them. (Sent. l. 2 d. 33 q. 2 a. 2)

5. Therefore, we arrive at the doctrine of Saint Thomas for adults. He, in the first place, prescinds from the fact and, abstractly speaking, affirms that to their only original sin corresponds the privation of the beatific vision.

   A defect that is transmitted by original sin, having the ratio of sin, is not because of any subtraction or corruption of a good which human nature attains from its own principles; but by some subtraction or corruption which was added to nature; nor does this sin pertain to this [particular] man, without according to his having such nature, of which this good which was in him was possible to be conserved, he was deprived; and thus no other penalty is due to him except the privation of his end to which the subtracted gift ordained [him]; to which human nature cannot attain. This is the divine vision; and thus the lack of this vision is the proper and only penalty of original sin after death: if other sensible penalties for original sin after death are inflicted, he would be punished not according to the sin he had; because the sensible penalty pertains to that which is proper to the person: that by his particular passion, such is the penalty. Wherefore, as the sin was not through his operation, so neither is the penalty through suffering due to himself; but only by his defect for which nature of itself was insufficient. For in other perfections and goods that human nature attains from its principles, the damned sustained no detriment for original sin. (Sent. l. 2 d. 33 q. 2 a. 1)

From this doctrine maybe it will seem to you to need to infer that if the adults observe the natural law and live well, as if they did not not understand revelation, they would be found in the condition of the aforementioned children. No! The doctrine of St. Thomas is more benign. Although he affirms in many places that if the adult, who does not know the faith, lives well according to the way mentioned (a thus never lacks grace) God, either with an interior inspiration or with an external succor, manifests to him that which is necessary to believe to save himself.

For having faith, one can prepare himself for that which is in natural reason; wherefore it is said that if someone is born in a barbarous nation, which is isolated, God reveals to him what is necessary for salvation, either by inspiring him or sending him a teacher. (Sent. l. 2 d. 28 q. 1 a. 4 ad 4).

But it is noted that, also in the doctrine of the Aquinate, that inasmuch as the man can observe the natural law, it does not exclude the common help of actual grace, although it does not include that proper of habitual and sanctifying grace; but he says:

But, however, the very fact that some do what is in themselves, namely converting themselves to God, is because God moves their hearts to good. (In Epist. ad Rom. cap. 10 l. 3).

All this doctrine counters the blasphemy that some ignorant people make against God of almost unjustly punishing men for sins that they do not commit. No! Each one who goes to the hell of torments goes because of his own personal sin. Together he mitigates the suffering of many mothers who are worried about the future lots of their stillborn children. Their condition is much better than that in which they would have incurred if their life was protracted and they died with grave personal sin on their soul.

Before Jesus died on the cross, the state or "place" of the souls of all dead people (Hebrew sheol, Greek hades and Latin inferus [1]) was divided into several "compartments".

• Gehenna or hell (Latin infernus [1]), inhabited by the souls of the damned, where they are tormented by fire (Mt 5:22,29,30; Lk 16:23-24). It is de fide doctrine that this is an everlasting state for each soul in it [2], and consequently that, as a potential state for souls or "place", it existed before the death of Christ, exists after it, and will exist forever.

• The "Bosom of Abraham", "Limbo of the Patriarchs" or "Limbo of the Fathers" (Latin limbus patrum), inhabited by the souls of the purified righteous before Christ's death, where they did not suffer any torment, did not see God, but were comforted by the expectation that at some future time they would be redeemed and taken to the vision of God (Lk 16:22). It is de fide doctrine that this was a temporary state for each soul in it, which ended after Christ's death, and that, as a potential state for souls or "place", it existed only before the death of Christ [3].

• The Limbo of Infants (Latin limbus infantium or limbus puerorum), inhabited by the souls of those who die in original sin only, where they do not suffer any torment, do not see God, and do not have any expectation of being redeemed and taken to the vision of God at some future time. It is de fide doctrine that this is an everlasting state for each soul in it [2], and consequently that, as a potential state for souls or "place", it existed before the death of Christ, exists after it, and will exist forever. (But it is not de fide doctrine that there are actually any souls in it, because it is not de fide doctrine that any person actually dies in original sin only.)

• Purgatory, inhabited by the souls of the righteous who still need purification, where they suffer a pufifying pain which is essentially different from the torment of gehenna. It is de fide doctrine that this is a temporary state for each soul in it, which ends when it is completely purified (after which the soul went to the "Bosom of Abraham" before the death of Christ and goes to the Beatific Vision after it), and that, as a potential state for souls or "place", it existed before the death of Christ, exists after it, and will cease to exist at the time of Christ's Second Coming and the resurrection of the dead.

Therefore, there are three possibilities for any adult person [4] who died before Christ's death:

• he went to hell (gehenna, infernus) and will remain there forever;

• he went, possibly after a time in purgatory, to the Bosom of Abraham, and after Christ's death was taken to Heaven (the Beatific Vision);

• he was still in purgatory at the time of Christ's death, and was taken to Heaven after completing his purification.

Notes

[1] There are two different Latin terms, which must not be confused:

• Inferus, meaning just "low" (comparative: "īnferior"), refers to the global state or "place" of the souls of all dead people, encompassing all of the above "compartments". This is the term used in the Apostle's Creed or Symbol of the Apostles when it states that, after his death, Jesus "descendit ad inferos" (plural accusative of inferus) [1.a].

• Infernus, a derivative word, refers to gehenna or hell [1.b].

[1.a] https://en.wiktionary.org/wiki/inferus

[1.b] https://en.wiktionary.org/wiki/infernus

[2] And, after the resurrection of the dead, for each whole human person, soul and body, in it.

[3] Pope Benedict XII in his 1336 Apostolic Constitution "Benedictus Deus" [3.a] [3.b] defined that the souls of all the righteous who were completely purified at the time of Christ's death were taken to the Beatific Vision at that time, and that after that time, the souls of all the rigtheous, once they are completely purified, go directly to the Beatific Vision. From that, it follows that, after the death of Christ, the "Bosom of Abraham" no longer exists as a potential state for souls or "place".

[3.a] http://www.papalencyclicals.net/ben12/b12bdeus.htm

[3.b] http://www.ewtn.com/library/PAPALDOC/B12BDEUS.HTM

[4] Cardinal Louis Billot SJ, in a series of articles published in "Etudes religieuses" in 1919-1925, proposed the hypothesis that a large number of chronologically adult persons remain, in the theological-moral plane, in the same situation as that of children before attaining the use of reason, because they have attained the use of reason with respect to the practical ends of life but not with respect to the ultimate end, so that they have not had the possibility to know God and thus turn to Him or reject Him, and that therefore the final state of those adults is also the limbo of infants. This hypothesis, if correct, would add a fourth possibility: the person went to the limbo of infants and will remain there forever.