Currently, when signing in with Apple, users can hide their email address, which means they sign in with an anonymous email address using Apple's private relay email service.  This article explains whether it is possible to force users to share their real email address when signing in with an Apple connection.

• Apple Social Login

The share/hide email behavior is documented in the Apple article How to use Hide My Email with Sign in with Apple. It states that the user can choose to hide or share their email address.

According to Apple guidelines - apps that use a social login service must allow users to keep their email address private as part of setting up their account. See section 4.8 in Apple's App Review Guidelines

As a result, Apple does not allow users to be forced to share their real email addresses based on their privacy guidelines. The user must be able to choose to share their email address or keep it private.