When you put something on the internet where it is accessible to some then it could become accessible to all. People think they're in 'private' Discord servers, meanwhile there's a bot in the same server scraping everything. Similar thing would happen with a relay.
https://www.404media.co/a-spy-site-is-scraping-discord-and-selling-users-messages/ 

 Yes, but that can happen IRL, too. There is no full expectation of privacy, anyway, but there are more and less private places. 

 A private relay is only as private as the users and operator of the relay want it to be