The API underneath the Win32 API, the Windows Native API, is not necessarily stable and therefore not intended for direct consumption by applications.

If you're utilizing undocumented APIs that aren't meant for public use, sure... but the core Win32 APIs have been pretty stable for going on 3+ decades now. You can take a lot of win32 apps from the early 90's and they'll run without modification in windows today... though, they'll probably run in Wine and likely a better chance there... but still.

I fully agree with that. Sadly there is always the odd application out there that uses the lower level stuff and is therefore tricky to get to run on Wine. Or more recent Windows versions for that matter.

win32 dates back to 1993. OP doesn't know Windows history. Maintaining backwards compatibility was always a huge priority for Microsoft, even if it couldn't be perfect.

If a program didn't work on a newer version of Windows, there's a good chance it was doing something unsupported.