Twitter started testing threaded replies in 2018.
Initially, efforts were limited to internal testing but the company rolled out the 'twttr' app to a selection of iOS users in February.
This was all supposed to make conversation threads easier to follow, but when it came down to it, Twitter discovered that didn't happen.
User feedback said the opposite, in fact - so, it's been scrapped, and so has the twttr test app:
Twitter launched the test app way back in March 2019, and that seemed to be as part of an effort to becomes more collaborative on product updates and tweaks.
All users got invited to apply for access, so they could offer their feedback on new features in testing. Twitter hasn't updated the app for ages, though.
Most users probably already forgot about it. It'll be interesting to see how the platform approaches testing in the future - because this truly didn't work!