Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
What about private messages? We should assume the person running the instance can read all private messages.
Yes. While I see no reason that private message would exist anywhere other than the instance of the sender and receiver, the admins of those instances CAN see the contents of the message and whether or not they have been read.
I’ve always assumed private messages on any site can be read by the site’s admin unless they are end-to-end encrypted.