It's probably worth at least a post to python-dev to expand the audience, but unless someone speaks up with some really good reason why they must update to Python 3.10 without updating their own code then let's deprecate it.

FWIW, even with the flag the behaviour changed in 3.6. Previously it would use the *A APIs and let Windows do the encoding, but then it changed to the *W APIs and Python would encode with our own mbcs:replace, where mbcs is just calling through to the WideCharToMultiByte API. So there isn't really anything to maintain long term apart from the additional options.