As far as I know, there already are encrypted DMs. Just the metadata isn't, so anyone can see who you are talking to and how often.
Discussion
That I recall. So I guess I should have phrased it as truly private chats or channels (group DMs).
Assume if we host a private relay for biz users (start ups) that would be a step.
XMPP is the other option for the chat part of the Nostr Slack.