https://techcrunch.com/2019/01/23/twitter-testing-original-tweeter-tag-to-distinguish-who-started-a-thread/

Twitter is testing a new tag that will make it easier to parse who started a thread. The new feature, which is starting to pop up for some users, makes it easier to find posts from the original tweeter within a thread, but may also help curb (some types of) abuse on the platform, making it easier to distinguish accounts that are masquerading as other tweeters, for instance.

Twitter confirmed the experiment to TechCrunch, noting that the tag has been rolled out to a “small percentage” of iOS and Android users across markets.