410

 410  is the HTTP status code for GONE, meaning whatever URL was requested is not there any more, and used in webmention CRUD to indicate that a source has been deleted.

"Any more" is important. A 410 response differs from a 404 in that a 404 states that the resource doesn't exist but lacks context on whether it might have existed in the past or future. A 410 states that the resource no longer exists, but did in the past.

That also implies that the current server has knowledge on the past state of the requested URL.

IndieWeb Examples
Examples of IndieWeb sites serving permalinks with status code 410 to indicate deletion.
 * uses 410 in both HTTP headers and tombstones' : https://evgenykuznetsov.org/en/reactions/2021/webmention-rocks-delete-1/

GitHub
GitHub uses 410 for removed features, e.g.:
 * https://github.com/snarfed/color/branches/references

http-equiv status workaround
If your website is hosted so that you can't control the status code returned by the server (one common case is Netlify), you can use  attribute instead.