A few examples from Civ 4/5/6:
Improvement Yield Bonuses
Civ 4: Distinct icons
Civ 5: Single generic icon for all kinds of effects
Civ 6: No icon - you have to open the tech tree and mouse over the empty space of the tech
Civics/Policies
Civ 4: Distinct icons
Civ 5: Distinct icons, but monochrome
Civ 6: One of four generic icons
Happiness/Amenities and Health/Housing
Civ 4: Icons!
Civ 5: Icon
Civ 6: Let's fill up the screen by writing out the word "Amenities" a dozen times
Oh hey, Civ 6 actually does have an icon for amenities despite not using it on that screen. It's cleverly designed to have the same color and shape as the Housing icon.
Improvement Yield Bonuses
Civ 4: Distinct icons
Civ 5: Single generic icon for all kinds of effects
Civ 6: No icon - you have to open the tech tree and mouse over the empty space of the tech
Civics/Policies
Civ 4: Distinct icons
Civ 5: Distinct icons, but monochrome
Civ 6: One of four generic icons
Happiness/Amenities and Health/Housing
Civ 4: Icons!
Civ 5: Icon
Civ 6: Let's fill up the screen by writing out the word "Amenities" a dozen times
Oh hey, Civ 6 actually does have an icon for amenities despite not using it on that screen. It's cleverly designed to have the same color and shape as the Housing icon.