some feedback from a happy fastmail customer: please add 2 factor authentication via sms! I neither have a smartphone or yubikey.
So I guess it's better to leave the choice to the user, with an appropriate warning text.
—Another happy Fastmail customer
A Yubikey costs less than a year's FastMail service.
So no, we're not leaving this choice to the customer sorry, SMS sucks for too many reasons to consider adding back.