I've yet to see one. It's one of the handful of things that don't exist yet on Lemmy that I'd actually miss from reddit. There's a few speciality subs that don't have any parallels on here but there's nothing stopping that content though. Porn on the other hand seems to be universally banned on every Lemmy instance I've ever seen. I guess moderating that content is just too much of a headache for most people.