Many commentators throughout church history have addressed this issue, though not all of them have done so while specifically citing or quoting the passage in question. Thus, for example, we can't necessarily agree with modern editors when they tell us that the Didache (1st/2nd century) and the Apostolic Constitutions (4th century) were referring to Exodus 21:23:

thou shalt not murder a child by abortion nor kill that which is begotten (Didache)

Thou shall not slay thy child by causing abortion, nor kill that which is begotten (Apostolic Constitutions, 7-1)

The same thing is true of Tertullian's famous opposition to abortion.¹ That being said, we do have several examples of both church fathers and later commentators dealing with the passage directly.

Augustine, Theodoret, and Aquinas

Augustine (d. 430) and Theodoret (d. ~460) both directly address this passage in series of answers to questions about Exodus. Augustine's full answer is not available online in English,² but his position has been summarized as follows:

Based on the Septuagint text, Augustine feels constrained to accept the view that abortion before formation is not homicide. [...] Early abortion is not counted as homicide because, while the soul may be present, it is in an insensible state.³

The reference to the Septuagint here is necessary because Augustine was using a version of the Bible that did not accurately reflect the meaning of the original Hebrew text. The Septuagint, translated into English, reads:

And if two men strive and smite a woman with child, and her child be born imperfectly formed, he shall be forced to pay a penalty: as the woman's husband may lay upon him, he shall pay with a valuation. But if it be perfectly formed, he shall give life for life⁴ [emphasis added]

Given this translation, it's not surprising that Augustine and Theodoret interpreted the passage the way they did. Theodoret provides more insight, particularly with respect to the case of the "unformed" fetus:

It is the general opinion that life is communicated to the fetus when its body is fully formed in the womb. [...] If the infant comes out with human features—that is, fully formed—the case is to be considered murder, and the guilty party must pay with his own life. But if it comes out before it is fully formed, the case is not to be considered murder, since the miscarriage occurred before the animation of the child. Nonetheless, the party responsible is to make recompense.⁵

From this understanding, perhaps aided by Aristotelian philosophy, developed the doctrine of ensoulment, that is, that fetuses do not gain souls upon conception. For example, though Thomas Aquinas (d. 1274) condemned abortion, he does not interpret this passage to mean that early ("pre-formation" and therefore "pre-soul") abortion is murder:

He that strikes a woman with child does something unlawful: wherefore if there results the death either of the woman or of the animated fetus, he will not be excused from homicide, especially seeing that death is the natural result of such a blow.⁶ [emphasis added]

Nonetheless, these authors agreed that at least for some fetuses, this passage teaches that abortion is murder. At least partially due to a faulty translation, these authors held that early abortions were not murder, and some made it explicit that they were instead finable offenses.

Reformation era

In the Reformation era and subsequent centuries, a similar understanding continued, but some Christians began treating this passage as referring to fetuses without qualifying their age. John Calvin (d. 1564) calls it a "great absurdity" to consider only the death of the mother a capital crime:

It ought surely to be deemed more atrocious to destroy a foetus in the womb before it has come to light. On these grounds I am led to conclude, without hesitation, that the words, “if death should follow,” must be applied to the foetus as well as to the mother.⁷

The Geneva Study Bible (1599) adopts a similar understanding.⁸ John Trapp (d. 1669) understands "no mischief" to mean that "no life be lost" but says:

There is a time, then, when the embryo is not alive; therefore the soul is not begotten, but infused after a time by God.⁹

Matthew Poole (d. 1679) applies the passage to both mother and child, but leaves the status of early fetuses as ambiguous:

"No mischief follow," neither to the woman nor child; for it is generally so as to reach both, in case the abortive had life in it.¹⁰

Summary

Unfortunately, not many pre-1800 commentators deal with this text explicitly and take a position, and even fewer do so in works that we have in English.¹¹ However, those that do agree that this Exodus 21 teaches, at least for "formed" fetuses, that causing the death of an unborn child is a capital offense.

References:

1. Apology, Chapter IX.
2. St. Augustine Questions on Exodus (French). See question 80 in particular, a portion of which is available in the work cited immediately below.
3. Soul of the Embryo, p119
4. Brenton's Septuagint
5. The Questions on the Octateuch, 48
6. Summa Theologica, Second Part of Second Part, Question 64, Article 8, Reply to Objection 2
7. Harmony of the Law, v3, Sixth Commandment
8. Geneva Study Bible, Ex. 21:22
9. Complete Commentary
10. English Annotations on the Holy Bible
11. For example, Adam Clarke, though born around 1760, published his work in 1832, so he is too late. John Gill is pre-1800, but merely marks the debate and does not take a position. Calmet's works are only available in French and Latin.