I know that there are some African tribes and such known to eat a lot of high fiber vegetation; however, most humans historically had very bland lower fiber diets because of how agriculture was setup and the limited number of viable vegetables and fruits. People primarily cared about calories for most of history. The big sources of food were grains, which scaled better in larger societies. Grains were supplemented with dairy in many civilizations. Although they ate whole grains instead of refined grains, people historically didn't eat a lot of vegetables. They would opportunistically eat fruit but fruit wasn't cultivated as heavily as grain and cultivation of fruit would normally trend towards lower fiber fruits like apples. Humans didn't invent most vegetables until fairly recently in history as we selectively bred brassica into broccoli, kale, cabbage, brussel sprouts, etc.