Saner apps defer the notification prompt till later, even if it's during onboarding after showing an explanatory panel first etc.

No, that's not the case. The user only gets a prompt if the app requests permissions. The user doesn't get a prompt if all the app does is check for authorisation – the state is returned to the app as UNAuthorizationStatus.notDetermined without showing a prompt.

If an app prompts the user for permission to send notifications as soon as it starts up, then it's the app's fault. It's been known to be bad practice (and counter-productive) for many years.

I do not think this is the case, I had a lot of apps require notifications permission at a separate page with a question asking the user if they would like to receive notifications from this app and pop up the system notification permission dialog after the user taps "I want notifications" button.