Really depends on the game's style and the type of foliage. Even DC has plenty of non-billboard greenery. Evergreens and such are often 3D, but bushier foliage tends to be made of billboards.Polygonal foliage just isn't very viable yet.
(Which highlights how little some aspects of foliage authoring have changed over the years. Plenty of sixth-gen racers do things the exact same way.)