Not sure if I understood you correctly, but the current requirements where implemented recently in our move to C++17 here: https://discourse.llvm.org/t/rfc-increasing-the-gcc-and-clang-requirements-to-support-c-17-in-llvm
I think you are right in your claim though. Say we gate building LLVM in C++20 mode we can’t make C++20 the default for a VERY long time.
Is there a specific C++20 feature that requires clang 17?