Think voting patterns, date/times active, submission categories, comments, writing style, punctuation ...
Facebook had a project to cross-reference user behavior to anonymized medical data. Say patient A's history says he visited a foreign country in the Mediterranean, came back with the stomach flu, was hospitalized for 3 days ... Maps to: holiday photos, dates, locations, status updates, friends commenting get well soon etc.
I hear they were pretty successful at it too! Makes you think what a government actor can do with enough data.
People generally don't seem to appreciate the relationship between marginal information and statistical likelihood. ~87% of Americans are identified by DoB, zip code, and gender. That was about two decades ago. The state of the art today is going to make Cambridge Analytica look like the Post Office when it gets pulled out of the shadows.
No you don't. You can leave the email box blank and sign up with no problems.
Just to be sure I just tried it via iOS Safari and signed up for a new account by entering a username and a password. Left the email field blank. Works like a charm.
Almost no other input flow allows you to hit “next” when the only form field hasn’t been addressed yet. Why not put username, password, and email on one page instead of splitting it up the way they do? Then you can mark the two fields as required and the third as optional.
Pretty easy to maintain a significant degree of anonymity by just deleting your account and making a new one.
With that said, most users are more concerned about being anonymous to their peers in conversation than they are to Reddit's administration itself, IMO.