Comment by edflsafoiewq

glibc 2.34 has a hard break where you cannot compile with 2.34 and have it work with older glibc versions even if you use those version headers. It will always link __libc_start_main@GLIBC_2.34 (it's some kind of new security hardening measure, see https://sourceware.org/bugzilla/show_bug.cgi?id=23323).

Since additionally you also need to build all your dependencies with this same trick, including say libstdc++, it's really easiest to take GP's advice and build in a container with the old library versions. And nothing beats being able to actually test it on the old system.