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.
This is what lemmy.world tells me when I want to delete my account:
"Warning: this will permanently delete all of your data from this instance. Your data may not be deleted on other, existing instances. Enter your password to confirm."
Edit: So if we want to own our data we should only post, comment and vote within our own instance or just keep in mind that whatever we do on other instances might be there indefinitely.
Regarding your edit: that will only help if your instance doesn't federate. If someone subscribes to the community on your instance, all actions (posts, comments, votes,...) are sent to all instances with subscribers and saved there.
That won't help AFAIK. For example, your comment seems posted on lemmy.world, in a lemmy.world community, yet I can see it on Kbin. If you delete all your data on lemmy.world I can still see it on other instances, since every instance has received a copy.
As usual on the internet, treat everything you post as public and irrevocable.
That is because of how ActivityPub works. Action is pushed to alle instances that subscribe to the community. Posts, upvotes, downvotes, comments, everything is also stored on all federated instances. There is no way to make absolutely sure that all servers delete your data.