you can find them on Mastodon instances too, in many cases. Deletes are really broken in ActivityPub.
Mastodon attempts to fix Deletes with LDSigs, but... that introduces problems of it's own because now there's a signed Delete message confirming that the thing you want to delete existed at one time.
Deletes could be fixed in all cases if we had authenticated fetches (because the origin would know *who* has the object!), and if Mastodon did not forward signed objects around the network.