NSFW should federate but no other threadverse platform supports NSFL so I doubt you'll see many of those. On other platforms they use nsfw for anything objectionable, including gore, intense war footage, gross medical conditions, etc whereas on PieFed we have the option of making a distinction.
(Posts from other instances that have "NSFL" in the title have their nsfl flag set automatically, so that's better than nothing)
At https://piefed.social/user/settings you can choose whether posts flagged as nsfw/nsfl show up in your feed. Communities dedicated to nsfw posts are not allowed on piefed.social but occasional nsfw posts can appear in any community depending on the rules there.
I'm probably going to add a more flexible mastodon-style Content Warning option pretty soon. However it's a similar situation to the nsfl icon - no other threadverse platforms would show it.