If you want to go this way, then take a look at compiler support matrix for last few language versions - if having an implementation is necessary for something in spec to "be a feature of C++", then C++20 and C++23 are still missing some features they're supposed to have.

IIRC it was present in Clang back at the time (which was relevant to us as we were using it); not sure about gcc or others.

Several features in ISO are yet to be fully implemented, there are compilers still catching up with C++17.

GC isn't the first one to victim of such lack of implementations, only EDG implemented C++98 external templates, for example.