Maybe a blockchain? A public and unalterable ledger (which is what a blockchain is, of course). But being completely unmoderated users will use tools to moderate for them, tools that can block content they don't like. Indeed, users would coalesce and build lists of who to listen to, and who to not, and they'd keep that list on the unalterable and uncensorable blockchain.
At some point, people learn that their lives are better without this or that group constantly coming in and will take protective/separatists measures. Which is exactly what you are seeing.
Now individual networks are private companies and might chose to have stricter moderation, but 1) that should place them in a different legal category, and 2) there should be enough competition to provide valid alternatives to the people being blocked from such networks, otherwise it risks abuse of dominant position, given the monopoly of access.
What both 'tgsovlerkhgsel and I are trying to say is that there is this kind of realpolitik going on at Mastodon, of which you should be aware before considering to join. You start going against the "party line", you risk getting defederated or, if you're an user on someone else's instance, having your instance's admins pressured to ban you on the threat of the whole instance getting defederated by others. And it's a significant threat, because as much as Mastodon is distributed, people still want to stay connected to the major instances.
I myself moved from Twitter to Mastodon a while ago and very much enjoy it, but I also try to not say anything there that would attract interest of the defederationists.