How to get red enemy unit outline in third person?

[Daggetd]

I've seen in some youtube videos where red outlines appear around enemy units in third person, making them much easier to see in cluttered environments.

And I saw this in the patch notes: "Added better visibility for units: Selected units maintain an outline, hovering over enemy uints gives outline."

Is this related? Or are the youtubers using a mod? Because I'm not getting the outline and I can't see the option in the menus.
I've seen "Unit Rim Lights" but that appears to do nothing noticeable?
How do I get the red outline? I'm not currently using any mods.

[M4F10S0]

Toogle on highlights in upper left corner. Now, you should see red outline in 3rd person.

[Daggetd]

That makes fighting in forests easier, thanks.