And for storing in a hidden sidechannel the DM unencrypted or encrypted by a second key in case u lost ur first, OTOH u could also instant repost any DM encrypted with the pub of a second seckey or later in bulk from ur relay .

Clients would only need to store all keys per social id they accept trusted too to see again ur old DM if they switch to ur new keypub.

Might need a nip-xx but not necessary.