Mastodon doesn't prevent censorship. If anything it makes it worse. Instead of dealing with professional community managers operating with clear rules, you are at the mercy of some random nerd running a server from his mom's basement. You can run your own instance, but then you're just talking to yourself. Nothing is stopping you from building your own Twitter, hell we had blogs for years before Twitter. You go to Twitter so you don't have to promote yourself to get an audience, and abiding by Twitter's censorship policies is the cost of admission.