Glibc has a terrible history of binary incompatibility. If that's so hard to believe, try running binaries built on one distribution on other distributions. Linux has two stable ABIs: the kernel ABI for static programs, and, ironically, WINE.
according to appimage recommendations as long as you build against glibc with an earlier version than the system it's run on it should be fine.
https://docs.appimage.org/reference/best-practices.html
I hear you about WINE though.
I haven’t heard of this and I don’t think you’re right. Glibc, for all its faults, as a general rule does backward compatibility well. The problem is if you compile against a newer glibc (common in CI by default) and try to run on a distro with an older (common in the wild). If your CI uses an older glibc you should be fine AFAIK.