NIP-36
Sensitive content warnings
NIP-36 lets a post mark itself as sensitive or include a content warning. The author can signal that the content may need care before it is shown directly in a feed.
Clients can then choose to blur, hide, label, or filter the post depending on user settings. It is similar to putting a warning label on a package before someone opens it.
Read more in the official 36.md NIP document.