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.
Not all details get propagated across the Fediverse, e.g. your email address only stays with your home instance, but there is information that get's propagated that may be non-obvious to a lot of people.
Not currently. Looking at the data it should be possible but I can't make an assessment of how much coding effort would be involved. Export wouldn't be a lot of work. Import would take a lot more consideration due to the need to reconcile data pointers and the effort involved in federating the changes, particularly when it comes to preserving timestamps.