I don't think there's widespread usage of GitHub comments on commits as a code documentation method, since it would be almost entirely unusable (you wouldn't be able to see those comments unless you navigated to the specific commit on Github).

The use case I do see sometimes is having a pull request made up of distinct commits which are each commented on, but that's already a bit of a poor UX in GitHub, easily worked around, and is pretty uncommon.

>Do you suggest that parent commenter should now go and change their policy on comments to solve this real immediate issue?

I mean what is your suggestion to OP? Putting their head in the sand and acting like Github didn't remove a feature they in fact had removed? It's a very niche feature (so much so that more comments seem to misunderstand the feature than actually miss the feature) so the chance of Github not removing it is fairly low even if complaining causes them to delay it a bit.