The problem is that "predictable reliable NRVO" is still a research problem. Real-world compilers do NRVO a lot but not in a way that is perfectly predictable — that is to say, not in a way that could be standardized across all compilers (or even between different releases of the same compiler).

A "perfectly predictable" algorithm was proposed in Anton Zhilin's P2025, back in the year 2021: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p20... but unfortunately it had some subtle corner-case problems (which I do not remember), so it was sent back for revision, and never returned with a fix. (Maybe because a fix wasn't possible; again I don't remember what the deal was exactly.)

The Right Path Forward would be for MSVC, GCC, and Clang all to try implementing P2025's algorithm in their front ends. Either something concrete breaks (reminding me what the problem was), or else all three mainstream compilers gain predictable NRVO and then we can "standardize existing practice." But the Right Path Forward requires tedious work by at least three people, which is hard.