I personally have never noticed this, to be honest.
I can understand that certain differences between iOS and Android can take some time to get used to, and now that you mention it I think that you may have a point that more visible notifications may be desired. However, there are still other ways that notifications can grab someone's attention: vibration and/or a notification sound. Perhaps AOSP designers think that majority of people will rely on those things.
Having said all that, changes like the one you're proposing aren't really the kinds of changes that GrapheneOS makes since the focus is on privacy and security. To make the proposed change and to maintain it in the future would likely take up too much of GrapheneOS developer's time and effort, leaving them with less time to focus the kinds of features and improvements that make GrapheneOS great.