The Linux libc API changed a couple of times in the 1990s (a.out → ELF, libc5 → libc6, etc), but the Linux kernel ABI (what libc uses to talk to the kernel) has

The Linux libc API changed a couple of times in the 1990s (a.out → ELF, libc5 → libc6, etc), but the Linux kernel ABI (what libc uses to talk to the kernel) has been stable since practically the beginning. Linus detests breaking anything that talks to the kernel and considers it massively unprofessional, to the point that even a terrible API will be set in stone if anything uses it.