These would have the property that when a person joins a platform, the platform can check what other users on it is controlled by the same person, without revealing the meatspace identity of the person in question; and without revealing the usernames to the trusted entity. (Well, ideally the platform would only get to know the number of users, not who they are; but I don't think that can be done.)
Then a platform could just put a hard limit on how many users any given person can have, while still retaining the users' pseudonymity. Or it could place a soft limit, for that matter: every subsequent user receives an automatic penalty (e.g. 10 downvotes on every new message).
It wouldn't stop state actors, and it'd be horrible in places where the government isn't held accountable, but otherwise seems to solve most of the problems of anonymity.
Perhaps you could replace the trusted entity with something peer-to-peer, but it would have to be very carefully designed.
