Apple originally certified Mac OS X as a UNIX because they used the word UNIX all over their website and the Open Group came knocking; they decided it was cheaper/better for goodwill to do the conformance and pay the license fee than to fight the case in court.
Looking now, I see almost no uses of the word UNIX on the website except the docs archive, so I guess they’ve decided to abandon both the certification and the use of the UNIX trademark in macOS.
The reason Apple went to the trouble was not really a marketing exercise, but rather because a lot of U.S. government contacts required it, or a waver. This is the same reason versions of Windows were also UNIX certified. In neither case did this certification actually make them more "standard" in the way that the rules originally envisioned.
So Apple's use of UNIX in marketing was to make sure they never ran into that as a disqualified on a government contract. My impression is that those contracts largely no longer require that (one small measure of common sense has caught up).
Maybe. But I don't remember seeing them use the word for the last decade, and the last-but-one release was certified. My first guess would be that it's too new, and hasn't passed certification yet.
https://www.quora.com/What-goes-into-making-an-OS-to-be-Unix...
It is strange now, but Apple made a big deal of marketing it as UNIX back then. Like at the front-page of apple.com. UNIX had a good reputation as solid and serious.