https://techcrunch.com/2020/12/03/twitter-finally-shuts-down-its-abandoned-prototype-app-twttr/

Twitter is shutting down its experimental app twttr, which the company had used publicly prototype new features back in 2019. The app was first introduced at the Consumer Electronics Show in January 2019, then launched to testers that March. Its primary focus had been on trying out new designs for threaded conversations, including things like how to branch replies, apply labels and color-code responses, among other things. Some of those tests eventually turned into Twitter features and the twttr app was no longer being used.

The idea to design in public was an interesting experiment by Twitter.