The new button will first show up on a few selected users on iOS, on the top right corner of the homescreen. Pressing on it brings up a menu that tells your current feed behaviour, and the two options for you to choose.
— Twitter (@Twitter) October 31, 2018 This is a good change, as it means no more digging into the depths of the app settings to change this option. On that note, Twitter is also testing other things, such as finding out what happens if the timeline reverts back to showing “top tweets” after the the app is closed or goes through a period of inactivity. No duration was mentioned with regards to the test, and there’s also no mention as to when Twitter will be testing this feature on Android. ,But considering it was about a month and a half ago that the announcement on bringing back the reverse chronological timeline was made, it may not be long before we hear about any new developments from Twitter again. (Source: Twitter)