One of sys._enablelegacywindowsfsencoding() issue is that os.fsdecode() and os.fsencode() are not updated, they continue to use UTF-8. Example on Windows:

>>> import sys, os
>>> sys.getfilesystemencoding()
'utf-8'
>>> os.fsencode('\xe9')
b'\xc3\xa9'
>>> sys._enablelegacywindowsfsencoding()
>>> sys.getfilesystemencoding()
'mbcs'
>>> os.fsencode('\xe9')
b'\xc3\xa9'

See bpo-29241 for larger issues caused by this function.

--

The first reason is deprecate this function is that it sounds dangerous to me and it doesn't seem to be used.

I only found one project which used it temporarily until they fixed their code to encode/decode filenames on Windows. It was used to workaround a in bug in their code.