This statement is not entirely accurate. When someone takes a screenshot of a regular direct message, photo, or video posted in chat, Instagram doesn't tell the user. There is one exception, though: if a user sends a photo or video that disappears after being seen once or in "Vanish Mode," the app will let the sender know if it is caught by a screenshot.
Later.com and other sources report that this has been the case for a long time and that Instagram has not added notifications for screenshots of regular conversations. A lot of people get the disappearing-media alert and ordinary DMs mixed up, which is why this tale travels so quickly on social media. In short, Instagram DMs can only show you if someone took a screenshot of a temporary media file, not a typical text or photo discussion.
