Nostur "Push" notifications work a bit different from regular Push notifications, it runs on your phone and periodically checks in the background if there are any new things to notify about.

Normal iOS Push notifications requires another service to run somewhere else to check for you and then push a notification to your phone.

The background method Nostur uses is a bit of a hack, it uses something called “App Refresh” that is normally used to make apps appear fast by fetching stuff before you have even opened the app. To save battery your phone learns how often and when you use each app and tries to refresh apps in the background efficiently, Nostur hooks into this to check for notifications.

Practically this means Nostur notifications could be a bit delayed, especially if you almost never open the app, but as a daily driver they work well.