IOS 9 or 10 (been a while) turned off SMS fallback when the user upgraded to that version. People kept calling and complaining texting no longer worked, and it was just that they could only text IPhones. That was 100% a dark pattern / monopoly practice with no reason but to ensure market share.
I'm sorry, but that doesn't seem accurate. I've been using iOS since ~2015 and I don't recall ever having lost the ability to send or receive SMS messages.
I had 25 IPhones all turn off the SMS fallback on the upgrade, and found Apple documentation for such. It was an intentional act, and happened over and over.
Well, if we are going for anecdotal evidence, that hasn't happened to me even once over the past 4 iPhone upgrades. SMS as fallback is enabled by default, and it has been this way for every single device I upgraded to so far.
If you left iMessage on when you upgraded from iOS to an Android device, everyone who had an iPhone would still be attempting to send you iMessages and you'd never get them on your Android phone.
If you downgraded to an Android device without first telling iMessage that your phone number was no longer on the iMessage network, then it would fail. This is unrelated to SMS fallback on an iPhone sending iMessages.
I saw, with my own eyes, an IPhone upgrade to a new major IOS version and the SMS fallback setting turn off. The change may have been walked back, but I saw it myself, and had to fix it on dozens of phones.
I have sms fall back turned off and I can still send sms. All that check box does ask you if you want to send SMS if iMessage is unavailable instead of automatically sending SMS.
