This issue was confirmed by a Twitch representative in a statement made to Polygon. “Due to a bug in the code that generated the message archive files, which has since been fixed, a small percentage of user messages were included in the wrong archives. The primary use case for Messages was promotion; streamers sending out mass communication to subscribers for example, and the majority of messages that were unintentionally provided to another user fall into that category.”
As Twitch notes, this only affected a “small percentage” of their users, whom they have since notified by email. “We have notified users via email and provided them the affected messages for review. Protecting our users’ privacy is important to us and we have taken actions to ensure this kind of error does not happen in the future.”
Unsurprisingly quite a few streamers weren’t too thrilled about the situation, even if their messages weren’t part of the leak as it basically highlighted how they had no control over the situation. If you want to see if maybe you were affected, you can check it out here on Twitch’s website.