Twitter's reply-limiting feature
"Before you Tweet, choose who can reply with three options: 1) everyone (standard Twitter, and the default setting), 2) only people you follow, or 3) only people you mention. Tweets with the latter two settings will be labeled and the reply icon will be grayed out for people who can’t reply. People who can’t reply will still be able to view, Retweet, Retweet with Comment, share, and like these Tweets."
**Update: as of 5/1/2024 you can also limit replies to "verified" users.