We've added the mode property to all Messaging API webhook events.
The mode property is necessary for the channel multiplexing feature (under development). Now, all existing webhook events contain "mode": "active".
The mode property's value can change to standby if the channel administrator starts using channel multiplexing. The value of mode never automatically changes to standby.
Learn more about the mode property by reading about the Common properties of webhook events in the Messaging API reference.