The C++ standard specifies it: https://www.cplusplus.com/reference/cuchar/
<cuchar> has not yet been implemented in libc++. See https://bugs.llvm.org/show_bug.cgi?id=44871 for the bug report, and the review for its implementation https://reviews.llvm.org/D97870 (this was submitted in March 2021, but hasn't see much updates recently).
*** Bug 263612 has been marked as a duplicate of this bug. ***
Has now been committed upstream in https://reviews.llvm.org/rG311ff3917827131c20d4b8dac4ce0394a2351b94