IIRC Discord gets around this by having a setting that needs to be enabled from another device/browser to be able to access NSFW servers on iOS. I think Pixiv (maybe Reddit too?) has something similar.
So I'm going to guess the workaround is that Apple requires NSFW content to not be accessible as a default, but does not get in the way for the content being accessible via some form of external switch. The 'external' requirement makes me think that maybe it's about shifting some sort of perceived liability? X might have to add a similar option I guess.