This is what the
web says
{username}/throttle
This topic publishes throttle warning and error messages to all subscribed clients. We do our best to provide an accurate accounting in the error message of the minimum amount of time you should wait before attempting to publish again.
Be warned: exceeding the rate limit too often within a short time span will result in a temporary ban from the Adafruit IO MQTT service.
You may only subscribe to your personal throttle feed.