Looking at the specification of the [[maybe_unused]], it states:
Appears in the declaration of a class, a typedef, a variable, a nonstatic data member,
A bug was logged for Clang and marked as resolved: https://bugs.llvm.org/show_bug.cgi?id=36231
This seems to confirm that the enum value is allowed to be missing in a switch without warning in case it is tagged with [[maybe_unused]]