Would it benefit the communities, or the public? Why should they be hidden?
As someone who is in their own niche communities (not on Lemmy) I suggest communities could have 3 modes: Standard, NSFW, and NSFL. If a community is marked NSFW or nsfl, it can never go back to Standard. If a community is marked NSFL, it can never be NSFW or Standard, or maybe if it does, all content from it is deleted.
Users could then set their preferences to standard, NSFW, or NSFL. If they choose NSFL, they see everything.