Anyone have a theory why this even made it to this point? the switch was such obviously a bad idea. just corporate weirdness that no one there bothered to raise their hand and be like "uh, are we really doing this?" or was there a story here that anyone knows about?
Maybe hidemyemail allows easily creating many accounts on a site. And some big site didn't like it.
The "Sign-up via Apple" button and creating an iCloud email yourself have a slightly higher barrier than creating a new throwaway hidemyemail email (1 API call w/o captcha/phone verification or whatever).
We might find out later this year if some site starts blocking @icloud.com but keeps allowing @private.icloud.com.
It's email reputation, it the always the answer with this kind of stuff.
My guess is that the bounce rate got too high and bot farms were using iCloud addresses like this.