@sheislaurence did:plc is spinning out to an independent org, relays are only necessary for things at scale (& aren’t used for user discovery), and relays currently cost $20/month for 42M accounts.
I presented at Fedicon last year about a selection of the many apps being built https://bmannconsulting.com/notes/beyond-microblogging-atproto/
For completeness, because of account architecture, ATProto doesn’t have a private data option today.