Re: embrace... extend... bloat?
If someone does not have a smart phone, can't come up with enough room to install something to help protect their accounts, or has such an old OS that the app won't run (added this one for ya) then 2FA of any kind is likely not something that is needed.
No smart phone? Likely not using online accounts that need to be protect via 2FA. If you need 2FA and don't own a smart phone, you should rethink your approach.
No room to install authenticator? You are too stupid to know how to use 2FA and are getting owned no matter what you do.
OS does not support authenticator? You phone is already owned and you are too stupid to know it.