Snapchat advertises that they detect screenshots. They try. But they're well aware that they can't actually know. For example, their bug bounty program policy

Snapchat advertises that they detect screenshots. They try. But they're well aware that they can't actually know. For example, their bug bounty program policy helpfully informs you that "screenshot detection avoidance" is not considered a vulnerability: https://hackerone.com/snapchat https://hackerone.com/snapchat . That's because it's always possible.