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.
kbin doesn't allow downvotes, right? I think the potential issue here is people figuring out who downvotes them then proceed to harrass the downvoters. Maybe lemmy should just store the cryptographic signature of the downvoter, which should be enough for the system to verify the action's validity, and skip storing the actual username.
I'm no expert but I believe comments are pulled down by an instance when a user requests the information and is then cached in the DB. So not everything is, but content that's viewed by a user on the instance will be.