>but relays that already protected DMs, have to turn that off or they can't serve concord groups.
This is not true. relay.ditto.pub gates giftwraps and nip04 DMs with AUTH. If you're in a Concord channel you hold the private key to the channel, so you just AUTH to it. It already works. Concord is perfectly compatible with NIP-17 relays except that NIP-17 doesn't explicitly state that the author shouldn't be allowed to delete. But it does say that the author should essentially throw away the key and that the recipient should be allowed to delete, so it's easy to argue that compliant NIP-17 relays _should_ prevent deletion by the author.
