Sorry, again this too vague for me. What is the exact problem with atomics and thread_local in C that would make the ABI dependent on a specific version of glibc? I know the ABI is not just calling convention and e.g. for atomics may involve calling a function from libatomic. But from my understanding, this is all part of a standardized ABI that does not change and can be provided by different implementations.

Then, what is the exact reason a library compiled against glibc must be loaded by a specific ld-linux? I could see that this is true for C++ perhaps, or when you use very special features, but I do not see this for C.

I often compiled programs against one version of glibc and run it against a different version, so I know there is not a tight coupling. So please be specific in explaining in what scenarios this would break.