Since your Tweet = your space, we've been testing new settings to give people more control over the conversations they start. Sometimes people are more comfortable talking about what's happening when they can choose who can reply. We’ve seen people use these settings to have conversations that weren't really possible before. Starting today, everyone will be able to use these settings so unwanted replies don’t get in the way of meaningful conversations.

These settings help some people feel safer.

People tell us they feel more comfortable Tweeting and more protected from spam and abuse.
Problematic repliers aren't finding another way – these settings prevented an average of three potentially abusive replies while only adding one potentially abusive Retweet with Comment. And, we didn’t see any uptick in unwanted Direct Messages.
People who face abuse find these settings helpful – those who have submitted abuse reports are 3x more likely to use these settings.
It’s a new method to block out noise – 60% of people who used this during the test didn’t use Mute or Block.

https://blog.twitter.com/en_us/topics/product/2020/new-conversation-settings-coming-to-a-tweet-near-you.html