Has the Anglo-Catholic understanding of suicide changed in the 20th-21st century enough to render Chesterton's comparison with martyrdom irrelevant?

The General Synod of the Church of England announced that suicides could receive Anglican funerals as far back as 2015.

Comments made in the linked article seem to reflect the general shift in attitude and increased understanding that the majority of suicides are no longer viewed as "self-murder", but a sad consequence of extreme mental anguish or mental illness.

However, I'm not sure this shift changes the Chestertonian view that you refer to. Chesterton was not simply talking about the suicides of depressed people. Your quoted text highlights a difference between, for example, Christians who died at the hands of persecutors, and those that chose to 'fall upon their own sword' instead.

Indeed, the article contains statements from various individuals within the Church of England that show that they were keen to make it clear this shift in attitude did not "endorse" suicide. The final quotation from one Bishop is particularly revealing:

"If we really believe that Samson did not commit suicide but died in battle, what on earth do we make of suicide bombers?"

The shift in attitude is one of compassion and understanding for those who give in to suicide as a result of extreme mental anguish or mental illness, but there is still a clear distinction between that kind of suicide and other forms of 'self-slaying' connected with martyrdom, as described by Chesterton.

Attitudes to suicide had already changed well before the time of Chesterton. The Christian attitude described by Chesterton was already in the past. This can be seen by his use of the past tense. The Christian attitude was ... not what is affirmed in modern morals. By modern Chesterton meant, of course, those of his time, the early twentieth century.

In the same chapter Chesterton wrote:

The suicide is ignoble because he has not this link with being: he is a mere destroyer; spiritually, he destroys the universe. And then I remembered the stake and the cross-roads, and the queer fact that Christianity had shown this weird harshness to the suicide. For Christianity had shown a wild encouragement of the martyr. Historic Christianity was accused, not entirely without reason, of carrying martyrdom and asceticism to a point, desolate and pessimistic. The early Christian martyrs talked of death with a horrible happiness. They blasphemed the beautiful duties of the body: they smelt the grave afar off like a field of flowers. All this has seemed to many the very poetry of pessimism. Yet there is the stake at the crossroads to show what Christianity thought of the pessimist.

The "stake at the crossroads" refers to the practice, up to 1823, of burying suicides at a crossroads with a stake through the heart. There are incidences of this still happening in early nineteenth century England. However, even a hundred years before that, the vast majority of suicide cases were judged to take place while the person was temporarily insane. If the person was temporarily insane then a normal Christian burial was held.

Whether a person was temporarily insane was not deternmined by a doctor or even a judge. It was left to a jury to determine temporary insanity.

In 1823 the law was changed and from then on even sane suicides were buried in consecrated ground, but only at night. The funeral service was still not read. In 1880 the general requirement to use the Church of England service at burials in Church of England churchyards was removed. From then on Nonconformists, Roman Catholics and others could have their own service, even though the burial was in a C of E churchyard. In 1883 this was extended to all suicides, and burial could be at any time of day. It was only that the official C of E service could not be used, which remained officially the case into the twenty-first century.

So when Chesterton refers to a stake in the heart he is talking of something that ceased almost a hundred years before he wrote. He does not seem to want it restored, and constantly refers to what he calls the Christian attitude in the past tense. They were not the attitudes of most Christians of his day.

Attitudes have changed further in the twentieth and twenty-first century but if anything this makes Chesterton's comments even more relevant. He was a great observer of paradox and is exploring the difference in attitude to suicides and martyrs, albeit not of his own day.

A further extract from the chapter goes:

An imbecile habit has arisen in modern controversy of saying that such and such a creed can be held in one age but cannot be held in another. Some dogma, we are told, was credible in the twelfth century, but is not credible in the twentieth. You might as well say that a certain philosophy can be believed on Mondays, but cannot be believed on Tuesdays. You might as well say of a view of the cosmos that it was suitable to half-past three, but not suitable to half-past four

Chesterton's contrast between "modern" attitudes and those of earlier times is all the more relevant now, to the extent that modern attitudes have softened further. Whether Chesterton would have written as he did if he had lived now, we can only speculate. But the paradox that he was exploring is perhaps even starker now than then when we think that the long ago Christian attitude was even more different from our own day.

Deliberate suicide is still officially regarded as a sin, but Anglican and Roman Catholic alike are even readier now to assume temporary insanity, and/or make the charitable assumption of a last moment repentance.