- one is about the format of symbol information in the actual ELF binaries which is only an issue if you are not using the standard libc functions for looking up symbols for some strange reason
- one is an issue that impacts targeting a lower version of glibc from a higher one which is a configuration that was never supported (though usually fails more loudly)
- the last one is a security policy change which is legitimately an ABI break, but mostly impacts programs that have their execstack flags set incorrectly
glibc actually goes to a fair bit of effort to be compatible with old binaries unlike most of the rest of the Linux userspace. The binaries I built for my side project back in 2015 (BlastEm 0.3.0) still work fine on modern Linux and they dynamically link against glibc. This is just a hobby project, not a piece of professional software, and a build from before this JangaFX company even existed works fine.
I find it really bizarre when people talk about Linux binary compat and then complain entirely about glibc rather than the sort of problems that the manylinux project has had to deal with. glibc is one of the few parts of userspace you can depend on. Yes, setting up your toolchain to build against an old glibc on a modern distro is a bit annoying. Sure, if you do something sufficiently weird you might find yourself outside what glibc considers part of their stable ABI. But from where I sit, it works pretty well.