Thanks for the swift reply Bo, I reported this to gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93161

So:
1 - P1946 was actually approved in November
2 - operator== (strong_order, strong_order) is part of the latest standards working draft (not not in the latest p0768).

Not a bug with gcc nor the standard. Likely one with MSVCs implementation, however P1946 is not yet listed on their conformance https://docs.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?view=vs-2019 table, so I will wait a bit before reporting it to them 🙂.

This issue can be closed, cheers.