§ 5.2. Accounts List indicates that email is required.

I am assuming that the purpose of the email field is to provide a human-readable identifier to help the user disambiguate when they have multiple accounts with the IdP (if there is some other purpose, that would be helpful context).

However, it seems conceivable that not all IdPs will use email addresses, especially in the future as email is becoming less common as a means of communication.

Therefore, I am wondering if email should be generalized with something like "human_readable_identifier" which the IdP can populate with an appropriate string: email, phone number, arbitrary username, etc.