This has nothing to do with security and backdoors. This is about a poor implementation of what should have been a sound service.
How I interpret it working and the problem.
You have a phone number 1234567890. You have an Apple ID
me@me.com
When you sign up to iMessage 1234567890 =
me@me.com.
So anytime you log in to a device using your Apple ID the iMessage system can send a message to your iMessage .
All good and dandy , we have a great product for all in the apple family.
Somewhere in the iMessage database there is a flag , something like iMessage_on = Y , so any messages you get from other users on iMessage will be sent over data and not SMS, to the Apple ID on the device.
Now comes the problem.
User takes out the sim from the iPhone , and places it into a device without iMessage support.
Your friend sends your a message on their iPhone , iMessage system sees the iMessage_on = Y , it sends the message to the Apple ID . PROBLEM - your no longer on an apple device, no message ever arrives. And cause your iMessage is on, you will not get it as a sms.
I assume the tool apple created flicked iMessage_on = N , off.
There are many different ways apple could have also addressed this, upon no delivery to target Apple ID after x attempts or x time, send a sms to 1234567890, as a fall back. If a user has not been logged into their Apple ID on a idevice after x time and keeps receiving iMessages , flick the indicator to N etc etc . Frankly these completely ignored the use case where messages were never delivered.....poor analysis and development.
How ever it's implemented , it does not take 2 years for someone to implement a unsubscribe functionality , it would seem apple deliberately took their time. Buying an iPhone again fixes the problem straight away, cause you have to sign into Apple ID .....
Though at the same time apple can turn around and say thier system works as intended in the apple ecosystem, and why should they have to develop for the use case where the user leaves . Some of us are approaching this as users who switch. Food for thought anyway .