main obstacle to a practical implementation of DMs on a network of intermediaries like nostr relays is not really any different from any other thing. you can do MLS using ephemeral events, as one way, though it has scale limitations, the way that doesn't is where relays enforce privilege to restrict reading private events to authed users. i have been hammering at this last point for the last 2 years but nobody seems to listen to me. fortunately, my relay implements it and there is a growing number of users who have in mind private network deployments that can also be easily bridged to open public access as well as respecting the confidentiality of the operators of this network. the liveness problem of private communication for methods like MLS make it a lot more challenging than it would be with a dedicated protocol, and i just think that the lack of DM support is the number one thing retarding nostr adoption.