Yeah. Even if they couldn’t hand over recovery emails, having a personal email as a backup to a “private and sensitive” email account is bad practice.
But what do you do if that field is needed? A throwaway address won’t work as it’s easy to recreate. Buy your own domain and run a server?
I don’t believe you need that field with Proton, correct me if I’m wrong. If you do need that field with an email provider, and you need complete opsec, use a different provider.
I put the Simplelogin email alias as my backup mail. Which forwards mail to my proton, so I guess it isn’t really a backup. Even more so if you realize I need to sign into simplelogin with my protonmail account and protonmail owns Simplelogin.
No, domain names are tied to a person and, even if that person register the domain with fake person details, there will be a digital payment associated with the purchase.