It will not break any valid program actually. I'm 97.5% sure that aborting or even throwing an exception on an out of bound vector::operator[] would be a fully conforming implementation.

The fact is that most standard library implementations can already be easily configured to bound check, so there is little pressure to mandate this in the standard.

The code, which would break, was already broken. Just in far worse ways than throwing exceptions: silent memory corruption. And there would be no backwards-compatibility breakage, since in cases, where it would throw, currently UB is triggered.