Hmm, doesn't seem to be explicitly listed as a deprecation, though discussion form around that time makes it clear that everyone thought it was.
I also found this proposal to use strict mbcs to decode bytes for use against the file system, which is basically the same as what I'm proposing now apart from the more limited encoding: https://mail.python.org/pipermail/python-dev/2011-October/114203.html
It definitely results in less C code to maintain if we do the decode ourselves. We could use strict mbcs, but I'd leave the deprecation warnings in there. Or perhaps we provide an env var to use mbcs as the file system encoding but default to utf8 (I already have one for selecting legacy console encoding)? Callers should be asking the sys module for the encoding anyway, so I'd expect few libraries to be impacted, though applications might prefer it.
Top-posted from my Windows Phone