Comment by kiwijamo

Can you cite an example of a big, well-known, etc service that enforces a 'you can't change your email address' rule? I haven't really seen this sort of thing in the wild to be honest. Maybe one or two over 20+ years? I think the norm is using a username or an internal identification number (such as an UUID). It is rare the email address is actually used as the primary key to identify an account.