Email in a JWT request

We’re in the process of implementing JWT-authentication for one of our apps instead of Anonymous authentication. We're using native support SDK for Android/iOS.

‘JWT Claims set’ of a JWT request should contain an email address, which, according to the respective article, is used as the unique identifier for a user unless an external ID is received.
It appears that “Email” is not an optional field, and a claims set should contain some email for a user to be identified.
Is it possible to use a placeholder email address instead of the one provided by a user with JWT authentication for it to work? Question is also brought up by GDPR-related concerns. 

Please advise. 

We're using the following documentation.