Ah, I wasn't aware that e-mail was used as an addressing system within AnyList. Do users not have any usernames associated with their e-mails?
Still, I think in this day and age, having a requirement in your product that says "e-mail that is provided should be the one the user uses the most" is pretty naive. In general, it's true, but when it comes to 3rd party authentication providers like Facebook, Google, and now Apple, this kind of requirement is not really useful and will likely cause issues for you down the line, which is why usernames are better for addressing people within apps (e.g, Instagram handles).
I'd personally for for the method used by Blizzard and Discord where a randomly generated ID is the actual unique value, while the username is just a display setting.
Still, I think in this day and age, having a requirement in your product that says "e-mail that is provided should be the one the user uses the most" is pretty naive. In general, it's true, but when it comes to 3rd party authentication providers like Facebook, Google, and now Apple, this kind of requirement is not really useful and will likely cause issues for you down the line, which is why usernames are better for addressing people within apps (e.g, Instagram handles).