GCC and MSVC are pretty close. fyi, the tables on cppreference are rather outdated at this point. I made a more up-to-date, community-maintained site: https://cppstat.dev/?conformance=cpp20
On the main page, it took me a minute after laughing at "std::byte arithmetic" to realize it was April 1st. All of those C++29 "features" are on point, very funny. Though surely there's somewhere SFINAE can be mentioned...
wow, that's weird. One would think that updating the reference table is something a team or individual - who just spent a lot of time and effort on implementing a feature - would also do.
For a while now cppreference.com has been in "temporary read-only mode" in which it isn't updated. Eventually I expect a "temporary" replacement will dominate and eventually it won't be "temporary" after all. Remember when some of Britain's North American colonies announced they were declaring independence? Yeah me either, but at the time I expect some people figured hey, we send a bunch of troops, burn down some stuff, by Xmas we'll have our colonies back.