This NIP enables authors to post exclusively into topics, without polluting their main feed. Authors can post to locations, hashtags, labels, and urls in a way that only followers of those topics will receive their posts.
## Topical Note Kind
Kind `21` defines a `kind:1`-equivalent event that MUST NOT be displayed unless the user follows one of the `g`, `t`, `r` or `l` tags.
```jsonc
{
"kind": 21,
"tags": [
["g", "<geohash>"],
["t", "<hashtag>"],
["r", "<url>"],
//..
["L", "<labelnamespace>"],
["l", "<label>", "<labelnamespace>"],
],
"content": "This is a note for followers of the hashtag, location OR url. My followers should not see this.",