It seems in iOS 17 that Apple have added a feature flag for Notifications under:

Settings -> Safari -> Advanced -> Notifications (API)

For me it was disabled by default and once I enabled it the pushmanager and Notification API was available in web applications without needing to add to homescreen.

I wonder why it have been disabled by default under a feature flag. Hopefully that will change in the future..

But otherwise ensure iOS 16+, valid domain certificate and add to homescreen