> Is this an optimisation or a change in program semantics?

Note that I specifically said something can be both an optimization and a change in semantics. It's not either-or.

However, it all depends on how the program semantics are defined. They are defined by the language specifications. Which means that in your example, it's by definition not a semantic change, because it occurs under the as-if rule, which says that optimizations are allowed as long as they don't affect program semantics. In fact, I'm not sure it's even possible to write a program that would be guaranteed to distinguish them based purely on the language standard. Whereas with tail recursion it's trivial to write a program that will crash without tail recursion but run arbitrarily long with it.

We do have at least one optimization that is permitted despite being prohibited by the as-if rule: return-value optimization (RVO). People certainly consider that a change in semantics, as well as an optimization.