Here's my two cents, as a non-expert when it comes to pathlib:

I'm not really sure why `is_mount()` exists on WindowsPath objects, given that it unconditionally raises `NotImplementedError` on WindowsPath objects -- that seems *very* strange to me. It seems to me like it should probably be a method on the `PosixPath` class rather than on the `Path` class.

The other methods that raise NotImplementedError don't seem as egregious, because none of them *unconditionally* raise NotImplementedError. We could debate whether some of them would raise different errors in an ideal world, but changing them now might have implications for backwards compatibility, and it doesn't seem like a *major* issue to me.