Currently, there is no webhook for when a queue item has been postponed.
We use the webhooks as part of our logging and alerting system around UiPath. We are trying to identify when a queue item is stuck, which we do by looking for queueItem.transactionStarted webhooks with missing queueItem.transaction(Completed|Failed|Abandoned) webhooks. However, there are cases when a queue item gets started and postponed several times before it does finally complete. In these cases, we are alerted that a queue item is stuck, but it is actually running just fine.
In order to fix our alert, we need to have a webhook we can listen to that tells us that the queue item was postponed.