Thanks for bringing it to the community’s attention. I had seen the author’s announcement on the openHAB community forum (where it has been well received) and suggested he do the same here.
That didn’t happen so I guess he is busy improving MQTT-Explorer! 
It already has most of what’s needed in an MQTT client but some of the suggested Enhancements, if implemented, will make it an even better tool for managing, and debugging, MQTT.