No, there is no guarantee that the reference will not be valid after the function returns. The only guarantee is that the reference will not be valid after the caller of the function modifies the referent (outside of an UnsafeCell), deallocates it, or creates a mutable reference to it. So there still is some escape analysis possible, in that any escape is scoped to the next modification.

Lifetimes do not affect whether or not something is UB on the language level; they only dictate which operations are safe and which are unsafe, and thus they act as a "social contract" for library-level UB.