This only works for 1:1 conversations. If you were in any iMessage groups and then deregister, iOS will silently drop you from future messages, and it isn't possible for the same group composition to be re-established over MMS.
SMS/MMS is insecure, so ability to replace an authenticated iMessage user with an SMS recipient in existing groups would endanger the whole groups.
It could at least ask the user the question. "One of the users on this thread has deregistered their number. Would you like to send by SMS so that they are included, or are you comfortable excluding them from this conversation? The SMS message would not be encrypted."
That makes sense since SMS doesn't support iMessage's always-on encryption but normally you can recreate the group using SMS once the former iMessage user's deregistration has gone through.
No, you can't do that. Once you've been a member of an iMessage group, iOS will not allow participants who have iMessage enabled to switch to MMS with that same group of users, even if everyone deletes and re-creates the thread.