AFAIK the issue is the symbol versioning which is explicit by design. I can understand why it exists for glibc-specific APIs but i don't see why it is also used for standard C and POSIX APIs that shouldn't change.

It is an issue if you want to compile new programs on a new distro that can run on older distros, but IMO that is much less of a problem than not being able to run older programs in newer distros. And there are workarounds for that anyway, though none of them are trivial.

Forwards compatibility is basically non-existent in the software world. It's not like you can compile a Win 10 program and expect to any time run it on Win 7.

Actually you can, as long as it doesn't use Win10 APIs (or use them via dynamic loading) it will work on Win7.

The issue with glibc is that when you use it it adds requests for the latest versions of the exported symbols that the glibc you use in your system has. You can work around this in a variety of ways (e.g. use __asm__ to specify the version you want and use the header files of an older glibc to ensure that you aren't using incompatible calls) but you need to go out of your way to ensure that whereas in Windows you can just not use the API (well, also make sure your language's runtime library doesn't use the API either but in practice this is less of a concern).

Actually you're right, I forgot that we used to ship software to various versions of Windows with just a single build based on more or less the latest VC++ runtime (that had re-distributable versions for all intended targets).