Well, inbox relays like that were that required AUTH for reading were my proposed solution for DMs and other secretive things since a long time. It's not only safer and more private, it's also more efficient for all the clients involved if they only read DMs from one place and publish to one or two places.
I was trying to get hodlbod (npub1jlr…ynqn) and VitorPamplona (npub1gcx…nj5z) to care about it and put it on the NIPs and get the clients to enforce a tag like "dms" on kind 10002, but I'm not sure I succedeed.