[fazfq]

What did you expect? Messages have to be stored on the server, and the instance owner has access to the server...

[scubbo]

(Hypothetically) wouldn't it be possible for client devices to generate key pairs, and for messages to be stored on the server encrypted in such a way that recipients' client devices could decrypt them? (I think that's what Signal does?)

Not saying that that's what happens on Mastodon instances, I don't know enough about it's operation to comment.

[remram]

Yes, end-to-end encryption is possible. It just needs support in clients, as well as a common protocol if you want it to work between different clients.

Mastodon has actually done some work towards that but I don't think it's useable yet, see https://github.com/mastodon/mastodon/pull/13820

[icelancer]

Normal users do not understand what federation is, much less how messages are stored.

[stormbrew]

This has nothing to do with federation. It's just a fact of life on any hosted internet service.