Websites block certain throwaway email domains from signups. The concern is that this will happen with private.icloud.com

A good example of a throwaway email that is now useless because of these blocks is mailinator.com. Originally, you could just make up a random email on the spot like gregsrightfoot@mailinator.com, visit mailinator.com, and get the needed signup verification email. These services autodeleted messages and required no signup so they were a black hole for spam. However websites eventually got wise that their spam wasn’t being seen and started blocking the domain. Mailinator came up with alternative domains and there was a brief back and forth before the throwaway email domains all ended up being blocked.