If I understood correctly, the standard doesn't specify
noexcept on functions with a narrow contract (with a precondition which violation leads to UB). N3279 and more recently P0884 are talking about this and about how to decide whether a function should be
noexcept or not (or conditionally).
This is the case for
pop_back but also on
back where there is no call to a destructor. Same for
back for example.