This looks like a specific account on an individual server. (which could have been registered by anyone) Each Mastodon instance chooses which other instances t

This looks like a specific account on an individual server. (which could have been registered by anyone) Each Mastodon instance chooses which other instances they can federate with (whitelist or blacklist). Given that Mastodon's early adopters included a large amount of people who switched because they thought Twitter should have banned more people (often because of perceived harassment), they tend to quickly ban or blacklist anyone they see as bad actors. It's free from censorship in the sense that someone could run their own server, but doesn't guarantee an audience.