Account recovery when the registered email address no longer exists

ADE doesn't encourage changes of email address. Of course, in the real world, those happen a lot. The one in the account I use for ADE hasn't existed for a decade. Now, I've found that ADE is heading towards two-factor authentication. So, I've tried to sign in to my Adobe account on the Web, and I can't do it, because it's sending a verification email to a long-dead address.

How do I recover access to my account and therefore keep my books, as I gradually replace my devices?