[AnssiH]

Isn't glibc, specifically, very careful about ABI compatibility?

You can still run Linux programs linked against decades-old versions of glibc on current systems as they have kept ABI compatibility with the use of symbol versioning, without changing their SONAME ("libc.so.6").

Sure, that does not extend to being able running programs linked against new glibc symbols on old systems, but if you consider that ABI-breaking then the Linux kernel would also be ABI-breaking as you cannot run binaries relying on new kernel syscalls (etc) on older Linux kernels.

[PixelOfDeath]

DebConf14: Linus about breaking librarie-ABIs:

https://www.youtube.com/watch?v=Pzl1B7nB9Kc&t=168s