According to runevision's comment here
Where he said:
"When writing the feedback site idea, please make it clear"
I will provide a CLEAR to-do:
1. Add an option to hide gray cube icon near each game object
Argument: We already know that it is NOT a prefab (text color is not blue). We don't need an icon for it. It is visual garbage.
2. Add an option to hide arrow on the right side near each prefab game object which opens a prefab.
Argument: Most people don't need to edit prefabs in isolation so often, standard "open" button is more than enough. This is a visual garbage.
3. Add an option to hide blue prefab icon IN CASE if this is a simple prefab without nested prefabs inside.
Arguments: Most people use simple prefabs without nested prefabs, they don't need an icon which will show when prefab have nested prefabs.
Note, to-do №3 should have 2 sub-options
Option A: Hide blue icon completely. Even if prefab have nested prefab. User will not be able to see that in Hierarchy window, but he will be able to see that in Inspector window anyway.
Option B: Hide blue icon only in case if this is simple prefab without nested prefabs inside, otherwise, blue icon will be displayed for this object.
This should be done. If you have any doubts, please read again two links I provided at the beginning of this text.