I would also like to know this. Additionally, is there an ID that never changes? Right now I am using the username as unique identifier, but if Twitch ever decided to allow to change this, it would really screw up my application.
“Twitch user data for users not created by your application may be stored for no more than twenty-four (24) hours.”
Interpreted literally, this means we can not store anything useful from Twitch like emails and usernames. We need more info. If we can’t store the username and other useful info for authentication, this makes the Twitch API nearly useless.