I've done all of that. The number is no longer associated with my Apple ID, but other iPhones still send messages to that number via iMessage — which means they go nowhere.
In the hopes of actually solving your issue, I hate to suggest this, but try reconnecting your number to imessage (and make sure you get any new imessage texts), then properly turn off imessage from that device. The defacto unplug it plug it back in.