I disagree with GP. I think email is generally a solid identifier, and would be curious to know why they needed to change theirs.
This happened to me, and I cannot use my comcast email with google services, and some others. I need a separate email address for job hunting, because google calendar will assume the provided email address is linked to google services, and the notifications will go to the other guy.
It is really fucking annoying.
Once someone has their own domain, it also opens up things such as hosting your own IdP (or paying a small monthly fee to have someone else host it for you) and sidestepping email entirely.
Sure, but requiring ordinary people to do this is essentially a nonstarter. The whole point of SSO is to minimize user friction. Requiring a user to also set up a special email account with another service is a dramatic increase in friction, and I expect that a large percentage of users simply won't do it. Why would they?