Instagram confirmed it has decided against the idea of alerting users when a screenshot is taken of their story.
Back in February I reported that this feature was spotted in testing, which was based on information posted by multiple users on Twitter.

Good bye freedom to screenshot IG stories. 💔 Thank you, Instagram for the warning.
— mulan (@__cajb) February 8, 2018

Adopting this feature would have put Instagram on the same page as Snapchat, which has always let users know when screenshots are taken and by who.
According to a statement given to Buzzfeed News, Instagram is no longer testing this feature and no one will get notified when screenshots are taken of their stories.
Instagram did not elaborate on why it decided to scrap this feature. Presumably it was due to the disappointment that followed when users found out they may no longer be able to take screenshots anonymously.
Users can now go about their business on Instagram, and feel free to take screenshots of stories without anyone knowing about it.