Rule 6, which prohibits promotion or discussion of personal engagement in illegal activity, is primarily intended to ensure that RF doesn't hold any legal liability for such posts. The default reference is federal law, which is why encouragement of, say, smoking weed is prohibited on the forums even though it is legal in multiple states. Support for legalizing an illegal action and discussion of the legality thereof is not against the RF rules, however.
If abortion became federally banned, RF would have to default to federal law in order to cover the forum's legal bases and protect it from any liability. However, discussions of the legality of abortion as well as support for legalizing it would still be allowed.
All of this is a preliminary answer, though, since judging such a situation and ruling on it would require further context and have more details and considerations that might not be apparent at this point.