Nostr.build checks for and removes CSAM (child pornography and other child related sexual material). They also have moderators which prevent any adult content showing up in the public galleries.

But people can use Nostr to post content on their own self hosted media server like blossom… they could also link CSAM hosted elsewhere on the web. 

There is a nip, or maybe a proposal that would let you upload files to a media server where the server operator can’t see the content… I’m not keen on that feature.