C (and C++) code breaks all the time between toolchain versions (I say "toolchain" to include compiler, assembler, linker, libc, etc.). Some common concerns are: headers that include other headers, internal-but-public-looking names, macros that don't work the way people think they do, unusual function-argument combinations, ...

Decades-old codebases tend to work because the toolchain explicitly hard-codes support for the ways they make assumptions not provided by any standard.