Apple's is up to 6mo old at release, so currently around clang10/11. Every so often it gets 2 releases behind as they are both on 6mo cycles. It's not bad really and currently Clang 12 brings few compelling reasons to upgrade. The issue will be Clang 13 is supposed to have a bunch of C++20 things, I think, and it will also be one of those ones that has a long time before Apple releases their next one.