(not sure how much Apple/Google even cared about the C frontend before though, but at least keeping the C frontend and stdlib uptodate by far doesn't require as much effort as C++).
Apple used to care before Swift, because Objective-C unlike C++, is a full superset from C.
Most of the work going into LLVM ecosystem is directly into LLVM tooling itself, clang was started by Apple, and Google picked up on it.
Nowadays they aren't as interested, given Swift, C++ on Apple platforms is mostly for MSL (C++14 baseline) and driver frameworks (also a C++ subset), Google went their own way after the ABI drama, and they care about what fits into their C++ style guide.
I know Intel is one of the companies that picked up some of the work, yet other compiler vendors that replaced their proprietary forks with clang don't seem that eager to contribute upstream, other than LLVM backends for their platforms.
However there are several C++23 goodies on latest VC++, finally.
Also lets not forget Apple and Google no longer are that invested into clang, rather LLVM.
It is up for others to bring clang up to date regarding ISO.