Notification expiry corresponds to the maximum period of time for which apple or Google servers will store and try to deliver the message.
You can set the expiry date when you send a notification from the dashboard:
or simply use "expiry" parameter using transactional and campaign API endpoints.
The value must be a duration from 0 to 2,419,200 seconds (28 days), and it corresponds to the maximum period of time for which Push provider stores and attempts to deliver the message. Requests that don't contain this field default to the maximum period of four weeks.