> Was pathlib forgotten here? Pathlib.home() is documented to return the same as expanduser("~") but it still prefers HOME instead of USERPROFILE.

Yes, it was forgotten (why doesn't it just use expanduser?). We should file a new bug for that.

> Note that this change has some effect on cygwin/mingw environments which all set HOME and now potentially lead to programs no longer being able to find their config files.

Firstly these are not supported environments, so it's not "rushed" for us to not preemptively consider them (though we'll happily merge most PRs that fix them without impacting supported environments).

And I thought the idea was that they'd use posixpath as os.path rather than ntpath? Cygwin in particular, which provides the full environment. MinGW is a bit lighter to be closer to normal Windows behaviour, which would suggest that using the Windows variables is preferable.