@mac.com no longer supported for Apple ID

A friend of mine just got a new iPhone 11 Pro. He was trying to login with his Apple ID (email) that ends with @mac.com. The iPhone will not take that email to login. He can login to the web using it and access his devices and preferences. When he contacted Apple Support he was told that he can only use @me.com or @icloud.com emails for Apple IDs. My @mac.com Apple ID is continuing to work just fine for me across all my devices (iMac, MacBook Pro, iPhone XS, AppleTV, etc.). I don't remember ever getting an email from Apple saying that they were going to deprecate @mac.com emails from being used as Apple IDs. I need to find out from him exactly how he contacted Apple Support about this issue but I figured I'd ask here first. Anyone ever hear of this?

-Doug