In game when you run the title command, there are options for title, subtitle, and actionbar. In the actionbar's case, the font has a background duplicate layer which is moved slightly and darkened. This is not in the vanilla game. See the two pictures, the first being completly vanilla, and the second being only forge (version forge-1.15.2-31.2.8, no other mods installed, same exact settings as vanilla)
For data packs that show your coordinates (which is how I found this out in the first place), it makes text that is black VERY hard to read. Especially something like < x:32 y:65 z:103 | North >, the numbers are very hard to read.
Vanilla. Notice bottom "Test" (Just above the hotbar) does not have the background layer
Only forge (Notice how "Test" on the bottom above the hotbar is darker/thicker)