I recently ported WebKit's libpas memory allocator[1] to Windows, which used pthreads on the Linux and Darwin ports. Depending on what pthreads features you're using it's not that much code to shim to Windows APIs. It's around ~200 LOC[2] for WebKit's usage, which a lot smaller than pthread-win32.
This is something you also need to do for other Win32 APIs, e.g. file write access may be temporarily blocked by anti-virus programs or whatever and not handling that makes unhappy users.
[1] https://github.com/WebKit/WebKit/pull/41945 [2] https://github.com/WebKit/WebKit/blob/main/Source/bmalloc/li...