That explains why they got it UNIX certified back then, but couldn't they stop advertising macOS as UNIX and stop getting it certified? They even changed the name from Mac OS X to macOS since then.

That's my question too, why continue to bother? Apple doesn't even have any separate "Server" OS anymore. I can't find anything mentioning UNIX on any apple.com marketing pages.

I guess it's just, might as well keep it going, as an option for future marketing if ever needed. Maybe it helps the salespeople in some enterprise deals? I mean, if it doesn't really cost anything to keep it.

My (wholly unsupported) guess is that there are government or megacorp bids somewhere for Unix systems for employees, and this checks that box. The buyer could update their requirements, but why do that when you can just make your vendor jump through the hoop?

Not at Apple and don't have any knowledge here, but I'd imagine that the UNIX test suite, ever since it began passing, has been a useful set of additional regression tests even outside the certification context.

Does anyone want to be the person that removes regression tests from active use, only to be responsible when something breaks that would have been caught by that test? Far easier to just fix your code so the test passes.

(And for many years, OS X then macOS had a reputation for being rock-solid, capable of going much longer betwen restarts, going into BSOD much less frequently than Windows would. Having a set of third-party tests certainly didn't hurt this!)

I obviously don't know, but I could easily imagine that Apples legal team has flagged it as a potential risk and the cost of keep the certification up to date is minimal, compared to some imagined risk. Safer to pay the fee, and not having to worry about someone at Apple accidentally calling macOS a Unix system in public.

Also, Apple is a huge company, there's the question of who's going to make the call the not update a certification that's negligible within the scope of macOS development. Better to not be that person and just rubberstamp the invoice from The Open Group. If management disagree, they can make the call, but they won't because the cost is to small for them to deal with.

there’s no downside as far as i’m aware.

I think it’s a quiet but deliberate strategy to keep macOS the spiritual successor to NeXTSTEP. While many of Jobs principles are under pressure at current day Apple, his ghost lives on.