I feel there could be specific webhosts dedicated to uploading explicit content, but many others may allow any kind of content but moderate or auto flag based on image detection models.

Here's a Tensorflow library that does this (https://github.com/infinitered/nsfwjs). Can be tested here https://nsfwjs.com/

I think lets say if NSFW content was published on nostr.build which now allows uploading images, audio and video files.

The host could add a similar library to detect such images and flag them / host them on another url ID and maybe Nostr clients can exclude such URL's.

However, since it's currently possible to upload images almost anywhere, this can be easily bypassed by bots attempting to spam explicit content. I think the best option is to auto blur images and videos in Nostr clients from non-followers or not load the content at all.