I don't see how the license is relevant here. It's not like companies have already implemented full C++20 support in their local forks and just haven't bothered

I don't see how the license is relevant here. It's not like companies have already implemented full C++20 support in their local forks and just haven't bothered upstreaming it. But yes, a lot of LLVM users don't care about C++ standard compliance at all, for example because they don't use C++.