Well there are still some c++20 items that aren't fully supported, at least according to cppref.

https://en.cppreference.com/w/cpp/compiler_support/20.html

Yeah, I think it's because none of the compilers are obligated to support the standard and things get added that never get implemented.

A good example is the C++11 standard garbage collection! It was explicitly optional but afiak no one implemented it.

https://isocpp.org/wiki/faq/cpp11-library#gc-abi