Being "invalid" doesn't nullify the backwards-compatibility rule - if the kernel accepts a certain kind of invalid ELF file, and people have come to rely on it

Being "invalid" doesn't nullify the backwards-compatibility rule - if the kernel accepts a certain kind of invalid ELF file, and people have come to rely on it doing so, then that has become a de facto part of the ABI. "But userspace is buggy!" is not considered a valid excuse. What does nullify the backwards-compatibility rule is no-one noticing or caring that it has changed.