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Disclaimer, This is only for Lemmy.world.
I actually read the privacy policy. There are basically 3 segments of data:
- The one time when you signed up.
- All times you log in, after you've signed up.
- User generated data
For part one: They store your username and the IP address used when you create the account. They store a hashed version of your password, not the actual password. They'll store that info for as long as you have an account with lemmy.world (although they reserve the right to keep it for up to 12 months after you've deleted your account). They store the hashed password so you can log into your account.
For part two: They keep a log of the times you sign in, the device you signed in from (iOS, Android, web) and the IP address you do it from. They delete this data on a rolling basis, every 90 days from the date the login data was created (from the time you logged in).
For part three: These are your posts, comments, upvotes, downvotes, etc. This is stored this until you delete your comment/post or undo your upvote/downvote. When you delete your account, if you haven't deleted your data, the connection (the association) between your account and the data itself is severed. This means that the comment will remain but the username value will be null.
tl;dr: I'm no expert but I think they keep a very small amount data. They probably do this to keep their costs as low as possible (but that is just my speculation).
If you're really worried about data mining and data logging, you can always go back to reddit /s