Letting Twitter users restrict who can @ them would be a pretty fundamental shift in how the platform works, preventing you from reaching out to a stranger through the platform to say hello or point something at them. Of course, it can also prevent bullying or harassment campaigns and give marginalized users another tool to protect themselves.
The screenshot posted by Wong shows controls that let you make sure everyone can mention you (as Twitter currently works by default), limit mentions to just the people you follow, or disable them altogether.
This isn’t the first time Twitter has let users limit the list of people who can interact with them on the platform. In 2020, it launched a feature that lets you limit who can reply to a tweet to just the people you follow or people you mentioned in the tweet, and the recent Twitter Circles feature lets you ensure that only a select few group of people can see certain tweets.