I've got a bizarre situation in which an OS upgrade bricked my Mac. I can't remember the details now, but it was something about an interface change in which I had managed to activate 2FA, but the login interface didn't have an input box for it.
The device was old enough that I just heaved a sigh and gave it to Apple for recycling.
> What if I use two-factor authentication on a device running older software?
> If you use two-factor authentication with devices running older OS versions – such as an Apple TV (2nd or 3rd generation) – you may be asked to add your six-digit verification code to the end of your password when signing in. Get your verification code from a trusted device running iOS 9 and later or OS X El Capitan and later, or have it sent to your trusted phone number. Then type your password followed by the six-digit verification code directly into the password field.
This weirdly works in some other places. Iirc one of the Amazon's seller login pages accepts the 2FA code appended to the password to avoid having to go through another page.
The device was old enough that I just heaved a sigh and gave it to Apple for recycling.