_GLIBCXX_ASSERTIONS is slightly different because it uses the bounds information that is already available. The historical expectation is that operator[] doesn't perform bounds checking by default, you need to use the at(size_type) members for that. Hence _GLIBCXX_ASSERTIONS for opting in. It's basically a library-only thing, much easier to implement than source fortification.
Agree the second one both sounds interesting and also quite disruptive to existing code.