If you choose an instance that has nsfw disabled, you can’t see those communities on lemmynsfw anyway, even via the search functions.

Instances that do have nsfw enabled needs to be careful as if one of their users goes onto an other instances community with illegal nsfw stuff, it’s cached on their servers anyway just like your browser caches the images and videos from websites. I guess the cache can be deleted on a cron job but still something I would wanna stay away from if I was a Lemmy instance admin. Defederating would be what stops this.

One of the greatest joys of federation, is de-federation.

The way I see it I think defederation is warranted in this case since it is potentially incriminating niche content. People don't expect to find porn on YouTube, there's Pornhub and the likes for that, so why would I want to see porn sprinkled into my regular Lemmy front page? Just create a porn account on lemmynsfw.com.

Also having spent 16 years on Reddit I simply know that there is content that I absolutely don't want to see, for example, scat and all things rape fetish. I get that there are people into that, but I would feel more comfortable knowing that these instances are opt-in rather than opt-out by default. Especially, because blocking entire instances is tedious for users.

Oh, lastly: I have no idea what the age requirement is on here, but from a legal point: There may be children around here too, and serving (especially unsolicited) porn to children is a huge no-no in pretty much any jurisdiction.