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.

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.