[whstl]

Yep, it uses an auto-generated @icloud.com for "Hide my Email" (useable in any website, or even if you want to give to someone in person) and @privaterelay.appleid.com when you use "Sign In With Apple".

This is quite visible in User Accounts where I work... while they do cause some issues from time to time (when the user disables the relay address for an active account), it guarantees privacy.

But I don't know if other popular single-sign-on provider do this.