But that's not new? Stuff like that could go into plenty of NSFW subreddits already, and faking celebrities heads on others bodies have been a thing since the dawn of digital editing (and maybe even earlier).

Regardless, I'd understand if that specific thing was made a bannable offense if that was the reason, but no reason to ban the entire niche.

Just like just because Photoshop could be used for what you're describing, subreddits about digital illustrations are not being banned.