This sort of thing is already becoming a reality. Not necessarily with Apple/Google IDs, but with email addresses. As the owner of a few dozen Internet domain names, I use some of them with catchalls so I can create new and unique email addresses for various entities I correspond with.

Sometimes this has failed, because the entity uses some third-party validation service that can find no record anywhere of the existence of the new email address. So it's sometimes impossible to register a new account somewhere unless you use an email address that is "known" to the validation system.

I'm not sure what "problem" they were trying to solve by doing it this way, but they've created a new problem by doing it.

Also, sometimes I've tried to use very short email addresses such as: x@xxx.com, and they're flagged as invalid even though they're not.

I've also had valid accounts disabled without notification because the email address I used had the name of the entity within. E.g. google@xxx.com Some companies assume you're trying to impersonate them if you do this, and silently disable existing accounts. Usually the tech support staff aren't even aware of these restrictions, which makes it even more difficult to recover.