This hasn’t happened yet, but what is the process if someone has lost/forgotten/eaten their U2F key?
How does one regain access or report the issue? Are there different capabilities for different plans?
Thank you from future me 
Have been thinking about how to implement a recovery key without sacrificing the security of the second factor.
In the meantime you can add a second U2F key to your account and keep it locked away in a safe place.