> "optimization" does not imply that only execution speed is impacted.

No but "optimisation" implies the normally observable semantics are not altered, which is not the case at all with TCE (aka "guaranteed TCO"). Removing TCE from a language makes it a completely different language e.g. Erlang would be severely restricted as it has no imperative loops.

Fair enough, but in most people's minds, it suggests that the correctness of an implementation doesn't depend on which optimizations are applied, and in this sense, TCO isn't an optimization.

TCO isn't merely an optimisation in the same sense of local/global value numbering/common sub-expression elimination , dead-code elimination, loop-invariant expression hoisting, etc.