This article talks about how custom emojis can be added to events in NIPS protocol. It explains that users can use the `emoji` tag with a shortcode name and image URL, which clients will then parse to display custom emojis. Additionally, users can add their own custom emojis by including the appropriate shortcode identifier. The article provides an example of how this works.

Link: https://github.com/nostr-protocol/nips/blob/master/30.md