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.
I think most of the users can understand the current situation. Take care guys and girls!