For the "hide my email" feature, Apple currently and has always used "@icloud.com". This masks you amongst all iCloud users.

They were planning to change it to a custom domain, which would allow sites to easily filter out and reject "Hide my email" users based on the email.

They have now reverted their plans to change this, meaning "hide my email" is still "@icloud.com".

I suspect many people actually feel better about Apple owning this fuckup and reversing it than they would have Apple hadn't fucked up in the first place.