Undefined behavior is 'anything goes'. An implementation can choose a particular behavior that you can rely on for a particular case of UB, because, if the only rule is that 'anything goes', it doesn't violate that rule.

I'll admit that compilers don't generally do that - because specifying it could lead to fewer optimizations. But I did say "if", and there's no reason they couldn't do so in principle.

One could imagine a compiler with an extremely strict debug mode that traps on a number of situations that the standard deems undefined behavior via a segfault, in order to help people avoid relying on UB. Again - saying something like "casting misaligned pointers causes a segfault on [system]" would in no way violate a standard that says "casting misaligned pointers can do anything", because segfaulting falls under the umbrella of anything.

I think you're misinterpreting the fact that the results of undefined behavior can be ignored by a compiler for a requirement that it must be ignored by a compiler.