What would that look like? How would this prevent me from returning a pointer to a stack frame that no longer exists, just for example? Clearly undefined behavior, but C doesn't seem capable of expressing this safely, with or without the compiler's help?
It wouldn't make it safe; it would make it so that the consequences of returning that pointer are just the same as if you had written an assembler routine returning that pointer. On all common hardware, that would be none if the pointer is never used; on most common hardware, it would be none if the pointer is never used in a call at least equally deep — but anyway, you'd get whatever the machine does, and if you don't like it, that's your problem. In pre-UB C, the ‘spirit of C’ that C89 was intended to maintain was that you get what you wrote.
By contrast, modern ‘undefined behaviour’ means that if you return that pointer — even if it is never actually used — the compiler can throw away all the code leading up to that point (including external side effects) on the grounds that it “can't happen”. You get much less than what you wrote.
How would this prevent me from returning a pointer to a stack frame that no longer exists, just for example?
It doesn't, but the outcome would be predictable: the pointer will always be pointing there.
I assume you mean by "stack frame that no longer exists" something like returning a pointer to a local variable; what that would do is return the address where the variable was --- the memory address still exists, so "no longer exists" is somewhat of a misconception here.
What "sane optimisations" mean is that the compiler won't e.g. decide to remove all the code following any call to that function, just because you "invoked UB".
Maybe you won't quickly find a practical and non-contrived application for this specific case, but there are plenty of others. UB breaking buffer overflow checks springs to mind.
