Microsoft sent out a message to Outlook.com subscribers over the past few days indicating that the Message History folder will be removed sometime this Fall due to the Skype integration. The message history folder currently records all online chat activity when using Facebook, Messenger, and Google Talk, allowing you to save and review past communications.

If you want to retain any old chat threads, you'll need to save them in a different folder. In the email, Microsoft provides the step-by-step instructions for creating a new folder and moving the messages you want to retain into it.