As a Black person, simply signing up for a Mastodon account can expose you to vile racist slurs and threats of violence. Most Mastodon users are one popular toot away from discovering that their instance mods are either unwilling or completely unprepared to deal with this.
Because a centralized whitelist was abused in a cynical attack years ago, the fediverse kinda gave up on that idea, and has been very resistant to it ever since.