even the newest clang does not support many of the c23 features.
gcc13 is much better, only a very few c23 are yet to be supported.
this is a long standing problem with clang/clang++: they're used in many linters and intellisense but they're lagging behind by a lot comparing to gcc.
Pure speculation, but I wouldn't be surprised if these two were directly related to each other. Clang/LLVM is more modular, which makes it easy to write new things that integrate it (like a linter), but this can slow down adding new things that change the data model and external interfaces. GCC is the opposite: one big blob that makes it easier to add things since the surface area is lower.
Almost all major compiler vendors have migrated to clang forks and if they contribute upstream, is on the backend side, for their platforms, not fronted changes.
Big contributors like Apple and Google, deciding to refocus on their own languages, and current language support being good enough for their LLVM use cases.