There's the other side of it, which is that SMS delivery is awfully unreliable in many places, and it's a frustrating experience for everyone. We can fire off the SMS, but if it fails to reach you it's an expensive and time consuming support headache for us, and a shitty experience for you.

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.