Apologies for only responding now, I've not received any notifications after my original pull request had been merged. I only learned about the change being reverted from https://github.com/python/cpython/pull/8580, so let me leave my two cents here:

I don't think the syntax not being valid (formally - since forever, practically - since 3.7) is good enough reason to make (lib)2to3 reject it. 2to3 is supposed to handle old syntax, isn't it? I'd argue that since it is (or was) possible to use this syntax in Python 2.x it should be handled gracefully.