Huh? AT Protocol is neither proprietary nor centralized?
Sure, AT protocol I agree. But Bluesky Social the practical entity as it stands today is a single, closed off instance, managed by a for-profit company. So I think it's still fair to say bluesky (but maybe not every piece comprising the production instance) is both proprietary and centralized. You can't run your own server, you may never be able to, and if you can it may never be allowed to connect with the "official" server, much like with Signal.