When inbound change sets become unavailable
Key Takeaway
An inbound change set becomes unavailable for deployment after it expires, after it is deleted from the source org, or after its source sandbox is deleted or refreshed.
An inbound change set is a change set that a target org has received: a set of changes uploaded from a source org. It waits in the target org until someone validates and deploys it, but it does not wait forever. Time passing, or certain actions in the source org, can make it unavailable for deployment.
The Inbound Change Sets page
The Inbound Change Sets page lists change sets awaiting deployment and the history of deployed change sets. To open it, from Setup, enter "Inbound Change Sets" in the Quick Find box, then select "Inbound Change Sets".