Well I'll chip in my opinion for what it's worth. My partner never had two step verification turned on, and she never tried or attempted to turn two factor authentication on. However, when she recently got a new iPhone (only a 5S so nothing brand new like the 6S) it was turned on for her. She has a 2015 MacBook Air as well.
I've been trying to get it for months since the posts appeared on the Apple Support Forum. I have spoken to them on the phone before and I've currently got my 2SV turned off, my iPhone and iPad are running iOS 9.2 and 9.2.1 respectively (no jailbreak) and my MacBook Pro (2014) is running OS X El Capitan 10.11.3 so everything is updated. The button for turning on 2FA is in the settings on all of my devices, but it gives the error message that states that "Two factor authentication is not available."
I don't know why this is happening but think I will phone up Apple again today to see. It's annoying, I want 2FA but feel more wary when I don't have any two step/factor on; currently you need my password only, but I don't want to turn on 2SV only to further set back when I can turn on 2FA.
Oh, also the other thing that I noticed in Apple's support post was that you can continue to use 2SV instead of 2FA, hence why I think it has to be off in order to turn on 2FA.