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.
That's in the US. In other countries things may be more secure. In Russia I barely can make any changes by phone because they verify so much info. Forwarding will certainly require an in-person visit to the carrier office.
So I guess it's better to leave the choice to the user, with an appropriate warning text.
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.
So I guess it's better to leave the choice to the user, with an appropriate warning text.
—Another happy Fastmail customer