@Ganymede I guess it depends on how the policy is set up. For the previously-mentioned Transformers game I once staffed on, the policy re: sexual themes was pretty clearly a matter of public acknowledgment. It was very much a "we don't police your stuff behind closed doors so long as you don't try to bring it out from behind those doors." Similarly, Arx's rule file regarding prohibited themes calls out that they're not interested in policing private RP that's kept private, but that they wouldn't be allowing storylines on those themes to be brought into the public sphere, other people's RP, or the game canon. Incest got added to that list of prohibited themes within the past year mostly because it was deemed to never be fun for players put in the position of having to be IC authority figures for these PCs who were engaging in a very big cultural taboo.
Do I think this means that private RP with incest themes has disappeared from Arx? 100% no. If a game just doesn't want certain themes brought out where anyone but the engaging parties can see them, does it at all matter if people are doing side things on Discord? Nah.