While mqtt is pub/sub and not request/response, there’s a trick we’ve added into the Adafruit IO MQTT Client which allows you to get the most recent value of a feed.
If you are subbed to username/feeds/key and publish to username/feeds/key/get then IO will send the most recent value to the client on the original subscription. The important fact is that username/feeds/key should be the same for both (you can’t have one use feed.key and one use feed.name)