Idea: a "Bitcoin backed relay" (I'll use BBR for short).

A BBR is a nostr relay that stores events on the Bitcoin blockchain using one of the blob store techniques (OP_RETURN, taproot envelopes, etc)

Saving a message here is _expensive_, you would not want to do it often, but comes with the upside of permanent storage on all BBRs (they all share the same backend state; like if nostr did relay gossip with decentralized spam protection)

This could be useful for notes that are very important. Notes that everyone must find. For example, perhaps a key rotation note, or some kind of deletion event. I could also see standard notes being here by people just looking for wider attention or posterity ("vanity notes")