Aren't notification LEDs somewhat obsolete now that we have always on displays? One advantage could be that they are less power hungry than keeping the screen / touch panel alive all the time. But in theory one could just create a permanent "notification LED" with an always on display, then it's the same thing from a user's perspective.
Smartphones are already insanely energy efficient. But instead of tuning them for longer battery life, companies kept pushing for faster processing, higher resolutions and refresh rates in the last few years. Now there's diminishing returns (imo 4k on a 6" device is just absurd) so I expect future devices to double down on energy efficiency. But then maybe companies just end up decreasing the battery's capacity for lower weight...