I prefer the name PYTHONLEGACYWINDOWSIOENCODING for symmetry with both PYTHONIOENCODING and PYTHONLEGACYWINDOWSFSENCODING, but I suppose you changed it to make it more visually distinguished from the latter.

I discovered this issue due to the modified behavior of PYTHONIOENCODING. It's not really a disruptive change, but it looks wrong based solely on the --help text description of PYTHONIOENCODING. An alternative would be to set Py_LegacyWindowsStdioFlag in _Py_InitializeEx_Private either when PYTHONLEGACYWINDOWSSTDIO is set or when PYTHONIOENCODING isn't a case-insensitive match for "utf-8" with an optional :errors spec. That way people get exactly what they asked for, even if it's mojibake nonsense given the console's current input and output codepages.