True when the delivery was in flight: CancelRequestedAt is set, the running handler's next heartbeat (30 s at most) aborts it with reason 'Cancelled', and the row becomes Discarded when the handler acknowledges — or at lease expiry if its worker is gone (03 §7).
True when the delivery is now Discarded, or (for an in-flight delivery) its cancel was recorded.
False when the transport cannot discard this kind of delivery (for example any SQS message).
Output of
WorkQueue.DiscardDelivery.