SMS is not very secure, there have been quite a few stories of people losing digital access due to an attacker just calling Verizon and taking over their phone account.
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.