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
![Click to resize (Javascript)](http://i.imgur.com/L0cI04R.png)
Civics/Policies
Civ 4: Distinct icons
Civ 5: Distinct icons, but monochrome
Civ 6: One of four generic icons
![Click to resize (Javascript)](http://i.imgur.com/QNEEif1.png)
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
![Click to resize (Javascript)](http://i.imgur.com/L3Cagen.png)
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.
![Click to resize (Javascript)](http://i.imgur.com/igJo7iQ.png)
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
![Click to resize (Javascript)](http://i.imgur.com/L0cI04R.png)
Civics/Policies
Civ 4: Distinct icons
Civ 5: Distinct icons, but monochrome
Civ 6: One of four generic icons
![Click to resize (Javascript)](http://i.imgur.com/QNEEif1.png)
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
![Click to resize (Javascript)](http://i.imgur.com/L3Cagen.png)
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.
![Click to resize (Javascript)](http://i.imgur.com/igJo7iQ.png)