Absolutely, we automatically persist notifications for offline devices which are delivered as soon as they reconnect, in the order in which they were sent. We refer to these as pending notifications.
This behavior can be fine-tuned on a per-notification basis using the time_to_live parameter in the Send Notifications API. By default, we try to deliver your notifications for up to a month, unless you specify a custom time_to_live value.
Note: On iOS, due to an Apple Push Notification Service restriction, only the last notification sent to the device will be delivered when it reconnects.
Only after all devices acknowledge that they received a push notification do we delete it from our database, unless its time_to_live expires before that.
If you have data privacy concerns, consider encrypting your notifications.