We (small HPC system) just upgraded our OS from RHEL 7 to RHEL 9. Most user apps are dynamically linked, too.
You don't want to believe how many old binaries broke. Lot of ABI upgrades like libpng, ncurses, heck even stuff like readline and libtiff all changed just enough for linker errors to occur.
Ironically all the statically compiled stuff was fine. Some small things like you mention only linking to glibc and X11 was fine too. Funnily enough grabbing some old .so files from the RHEL 7 install and dumping them into LD_LIBRARY_PATH also worked better than expected.
But yeah, now that I'm writing this out, glibc was never the problem in terms of forwards compatibility. Now running stuff compiled on modern Ubuntu or RHEL 10 on the older OS, now that's a whole different story...
You don't want to believe how many old binaries broke. Lot of ABI upgrades like libpng, ncurses, heck even stuff like readline and libtiff all changed just enough for linker errors to occur.
Ironically all the statically compiled stuff was fine. Some small things like you mention only linking to glibc and X11 was fine too. Funnily enough grabbing some old .so files from the RHEL 7 install and dumping them into LD_LIBRARY_PATH also worked better than expected.
But yeah, now that I'm writing this out, glibc was never the problem in terms of forwards compatibility. Now running stuff compiled on modern Ubuntu or RHEL 10 on the older OS, now that's a whole different story...