For your protection, we have expired your password and stream keys. In addition, if you had connected your account to Twitter or YouTube, we have terminated this connection.
You will be prompted to create a new password the next time you attempt to log into your Twitch account. If applicable, you will also need to re-connect your account to Twitter and YouTube, and re-authenticate through Facebook, once you change your password. We also recommend that you change your password at any other website where you use the same or a similar password.
We apologize for this inconvenience.
The Twitch Team
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I posted the email text I received instead of the notice on their blog:
http://blog.twitch.tv/2015/03/important-notice-about-your-twitch-account/
The email version includes more specific information on what was leaked (personal information on top of e-mail address/password).