The discovery was made by a developer, who was able to dig up the Vertical Stories feature from Instagram’s source code. Of course, since it isn’t still live, we can’t see it in action yet.
— Alessandro Paluzzi (@alex193a) February 2, 2021
The Instagram representative even confirmed to TechCrunch that the feature is in development, but said that it’s still an early prototype and as such it’s not available to the public. When that might change wasn’t revealed, unfortunately.