As part of Twitter’s “Your Tweets = Your space” initiative, Twitter users will now be able to change who can reply to their tweets even after they post the tweet.
— Twitter Safety (@TwitterSafety) July 13, 2021
Previously users would have to make the decision before they post the tweet, selecting between Everyone, People you know or Only people you mention.
Twitter says the previous rule cut down significantly on abusive replies, while only leading to a slight increase in abusive quotes.
The feature appears to have rolled out to the mobile app already but does not appear to have reached the web app yet.