I believe apps can choose to hook into "sensitive content warnings" (https://developer.apple.com/documentation/sensitivecontentan...) but I don't know whether any of the major social-networks-with-a-lot-of-porn apps actually do so.

Mostly, they're in the App Store with age ratings of 17+, so parental controls will block them out. No idea how that system interacts with third party app stores, either -- but if it doesn't work there, that was very much Apple's implementation-choice.