Despite its popularity with its users, Snapchat has been struggling, being overshadowed by competitors like Instagram, who have unapologetically borrowed some of its features for its own use. So it comes as no surprise that Instagram is once again borrowing another one of Snapchat's features. Users will now be identified if they take a screenshot of public Instagram Stories.
When a viewer takes a screenshot of a person's story, it will notify the person that posted the story that a screenshot has been taken and by who. While the feature is live for some users, according to Instagram, it is only currently testing it, so if you don't see it in your app, there is no need to be alarmed. If you are one that screenshots stories, you'll first be warned with the message above, before users are notified that you are taking a screenshot.
As stated previously, this mode is currently being tested, so there is the possibility that it won't have a full-fledged rollout depending on the feedback it receives.