On slidestr.net I use hashtags and content warning NIP to identify adult content - I still had to mark some profiles (npubs) as NSFW because that was not reliable enough, when neither is available.
The content warning NIP could be a solution but it has does have some issues....
- As a content creator wants to be visible to users for higher engagement and not be hidden behind a second click.
- It is very subjective what is acceptable and what should hidden behind a content warning.