No, sorry I definitely was not clear enough.

What I am saying is that you seem to be assuming the primacy of Posix. For example, how would I write a platform agnostic version of the following with your AbstractPath?

class TaggedPath(AbstractPath):
    def add_tag(self, tag):
        ...

or would it be:

class TaggedPath(AbstractPath, WindowsPurePath):
    def add_tag(self, tag):
        ...

Does it make sense what I am trying to point out?

The reason that Path isn't subclassable is that it is a factory which is dependent on its two subclasses, and upon instantiation a decision has to be made about what class to return, PosixPath or WindowsPath.

If I am understanding correctly it seems like your solution with removing flavour from AbstractPath ignores that Windows-syntax paths exist. If you ignore that Windows-syntax paths exist, then of course there is no decision to be made.

As such, to me this doesn't really seem to working towards solving bpo-24132 because if the user in the original post was just assuming a specific platform, they could have subclassed either PosixPath or WindowsPath and had no problems. The trick is to make it work while not assuming a specific platform. Maybe I am missing something, but I don't understand how that will be possible if you strip out the Windows-style path syntax into PureWindowsPath like you did in these commits.