Mod like that is already exist. It's call Busy Laborers. The mod add buttons to allow you to tell laborers to collect wild food, and herb just like you do with stone and iron.

Old myth of the old growth forest. Herb doesn't "require" old growth forest, just need mature tree.

Old growth forest seems to yield more herb (and wild food) because the number of tree is optimal (no added tree by the foresters), and most of the tree are mature. The forest reach the optimal balance between tree, food and herb. There is no foresters there to replace old tree, which herb need, with young tree, which food can grow but not herb. Over time, the forest get out of balance and in favor of tree and food because of the foresters' work.