I’m not convinced by this argument: at the back of the phone is a built-in LED (used as the flash). Which could be used for notifications too.
Phones with OLED screens could use part of the screen as a notification as well. Both of these can be accomplished in software. Currently you have to notice that something happens as it happens, otherwise you need to at leas activate the screen. The notification LED was useful in that you could glance at your phone and see if you missed something.