Twitter heavily censors a large number of their users. And, despite what you might have heard, it's not just conservatives. They censor all types of users: liberals, conservatives, sports fans, music fans, celebrity fans, and on and on. Despite what Twitter says, it's not just a small percentage of users, and it's not about bots. In fact, Twitter tends to elevate bot-like replies (like lone pics).
For instance, on most threads to U.S. government or elected officials' accounts, around 20% to 50% of the tweets are censored: