Can you give flavor on this: "Now we need to remove our phone number as backup method. (If you're curious why it's important to not have a phone number on your account, see the (https://techsolidarity.org/resources/security_key_faq.htm)[s... key FAQ].) "

Unfortunately, the security key FAQ contains no info on why you shouldn't have your phonenumber. I assume it's because phone-number migrations can be used to take control of your accounts?

EDIT: Yeah. Perhaps reading the article instead of just coming to the comments might have been useful.