Basically it seems that the imessage app on iOS basically can't be bothered to check every number every time a group message is sent. So once it has found you were a imessage number you are forever an imessage number within that group.

The only way to break it, as best i know as an outsider, is for everyone else in that group that are imessage users to delete the group and start over.

Likely it is another one of Apple's "usability optimizations", like how they would broadcast every last SSID you had been in contact with to try to speed up reconnects.

But Apple Central knows that the number is no longer on iMessage. It also know that I was the recipient of an iMessage in the group. How difficult would it be for Apple to send an update to the client, saying "Um, you tried to iMessage $NUMBER, but it is no longer on iMessage" ?

As best i can tell, what you suggest happens on one to one messages. But there seems to be something in the way iMessage stores group messages, and use whats stored when responding, thats the problem.

If i am to guess, the client-server protocol only sends a single message id, and thus when one part of a group fails, the server has to invalidate the whole group.

And Apple would be loath to do that as it would perhaps be seen as an inconvenience to the user.