YSK: Your Lemmy activities (e.g. downvotes) are far from private
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.
Does anybody know if your subscriptions can be seen by admins of other instances? It doesn't seem like that information would need to be shared, but maybe it is anyway.
Yes. When you subscribe to a community you send a Follow Group request which updates the list of Followers. I believe only your instance's admin and the community you are subscribing to's admin can see this though.