I think it's become one of those phrases that are shorthand for something else, that don't necessarily have their literal meaning anymore. I think people use it as synonymous with "porn", "risqué", or "not for minors". I don't use it in my content warnings on Mastodon or my website anymore, preferring to list the actual kinks and potential triggers on display instead, since yeah, "NSFW" on its own doesn't really explain much!

I think in general "NSFW" is shorthand for "things you wouldn't show to vanilla, non-kinksters you don't have a very close relationship with, whose tastes you don't know and who haven't consented to being shown these things".