Windows API and ABI has always been more portable than anything else. This is why Microsoft is a tech giant. You can take a windows binary from 1995 (actually even older) and run it reliably today.
If it doesn't run and you are a commercial client, Microsoft will implement a compatibility hack for you in the latest windows code so your thing from 1995 will work.
There is no parallel to this in the tech world so far. Linux gets around this by requiring you to recompile things, but recompiling old code along with old compilers and old libraries and all their dependencies is a nightmare.
Sure, I know all that, but it’s ironic to me that Unix, which was boasted as the epitome of portability once with C, POSIX, X11, X/Open and whatnot, actually struggles with backwards compatibility while Microsoft, the notorious developer of a closed-source, locked-in platform, has managed to become the key to portability :)
That's source-level portability, while this is binary portability. Even early on, MS put in lots of effort to ensure that old applications kept working.