When reddit first added fake/branded posts as ads for a short while it was possible for people to post on the branded posts. It was hilarious (vicious roasting of the products and brand) but of course that was quickly locked down.

I was a fairly early Birdwatch tester; they definitely permitted political posts to be annotated. I can't recall either way on ads; given Musk never made a big "I'm enabling it on ads!" announcement I suspect it was always possible.

(Especially considering you can promote a post after the fact; it's not necessarily an ad to start with.)