Obviously, they didn't show her having the procedure. So it's ambiguous.
Well, I think this makes it more than just ambiguous, it makes it more likely than not that it didn't happen. On a show obsessed with historical details
it seems likely to me that they would have shown what a pre-Roe abortion procedure was like (this is where my wife thought the scene was going). And Joan certainly conveyed misgivings about going through with it, first with Roger in the coffee shop and then in the waiting room where the woman assumed Joan had a daughter. It also was convenient that neither Roger nor anyone else from the show was present to witness what did or didn't happen. It just seems to me that the whole thing would have been written in a different way if she had actually had the abortion.