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.
If you are doing anything tgat could get you in legal trouble on the internet, only use acounts that can not be linked to your real life identity, and always use tools like Tor. Do not depend on tools like private messages, private voting, etc. In those cases, there is always someone who can give you away, and service admins will give out information when the feds come knocking.
The issue isn't about any one person... so many people here are making it a personal problem. The problem is the ability for literally anyone to track how ideas propagate PER USER. The person analyzing this data doesn't care who any one person is in real life.
Add a matrix account to your profile and any time someone DMs you, remind them that DMs are not private and guide them to your Matrix. Then pretend to disagree with them energetically on DMs while sharing your terrorist plans on Matrix.