> glibc dependencies that can't be resolved because you need two different versions simultaneously in the same build somehow...

If you somehow experience an actual dependency issue that involves glibc itself, I'd like to hear about it. Because I don't think you ever will. The glibc people are so serious about backward and forward compatibility, you can in fact easily look up the last time they broke it: https://lwn.net/Articles/605607/

Now, if you're saying it's a dependency issue resulting from people specifying wrong glibc version constraints in their build… yeah, sure. I'm gonna say that happens because people are getting used to pinning dependency versions, which is so much the wrong thing to do with glibc it's not even funny anymore. Just remove the glibc pins if there are any.

As far as the toolchain as a whole is concerned… GCC broke compatibility a few times, mostily in C++ due to having to rework things to support newer C++ standards, but I vaguely remember there was a C ABI break somewhere on some architecture too.