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.
Can you link to the apple documentation? I've never experienced this phenomenon you are describing, nor has any of my friends & family.
Probably not since it likely does not exist. Will update or delete this if they do follow through.
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.