Coming from the S7 after 2 years, I'm not really noticing any difference in terms of notification volume on the S9+.
One thing I've noticed however is that the ringtone volume coming from the earphone part (top of phone) is a lot softer/quieter than the volume coming from the bottom firing speaker - now this is expected and normal I think, but here's the issue I've noticed - the ringtone volume is even more soft/quiet when an actual call is coming through compared to when testing it in Settings. Can anyone else confirm this? You can test it by covering the bottom firing speaker with a finger/thumb and listening to how loud the top speaker is when the ringtone is playing.