Maybe play around with this community. It's a NSFW community with some untagged posts in it.
In my experience, if a user has selected to not show NSFW, then posts in a NSFW community won't show to them (regardless of how the posts are tagged). I've also seen the inverse: NSFW posts from an untagged community with an account that wasn't supposed to see them.
How things are actually display though, is very dependent on what the user is using. They tend to err on the side of caution though (e.g. Voyager blurs everything from lemmynsfw, even the 'celebs' posts that are photos of fully-clothed women)
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