P2167R3 Improving boolean-testable Usage

No feature-test macro is mentioned.

It's unclear to me whether there's anything for implementers to do here, or if this is merely a specification cleanup. If we determine that there's nothing to do, we should remove the cxx23 label, as this was never a feature to begin with. (Edit: It appears that changes do need to be made - thanks @frederick-vs-ja!)