Right, I guess other limitations need to be considered for them then if they're ever included.
Personally I'd like to see a distinct division between hunter-gatherer societies, nomadic societies, agrarian societies, and complex societies. To take the Mediterranean as an example, agriculture started to catch on circa 10000 BCE, but it wasn't until circa 5000 BCE that developments in fermentation, tanning, permanent settlement, crop and animal domestication, advanced pottery, irrigation, smelting, sailing, and other technologies over the last half millennia compounded into sufficient population growth to necessitate the development of more complex societies based on a specialization of labour.
DOC usually represents agrarian socities as "uninhabited land", the problematic nature of which I hope speaks for itself. At the same time, it is a representation born out of the limitations of Civ 4 as a system, as tiles are either inhabited by a local settlement, controlled by a distant settlement (in some cases like Bronze Age Greece this control is solely cultural in nature), empty, or solely occupied by a unit representing members of a nomadic peoples, and each city adds to the complexity and thus the resource requirements of the simulation. To allow every tile to have its habitation properly reflected, you'd either need to create a new type of more simplistic city mechanic or represent such cultures solely via units, representing all but the most notable in a manner similar to the independents.
A more practical implementation would be to add such societies as civs, but to hamstring their population growth, production, and commerce, perhaps by restricting them from gaining bonuses from Resources, such as Farms' +2 Food with Corn.
I used terminology from the game. Besides, I named them because I remember them, and I remember that they are shown as barbarians.
Yes, but just because the terminology is in the game doesn't mean it's not problematic. Rhye's and Fall games, and history simulators in general, are not "turn off your brain" type games. You role play as imperialists, colonists, and all sorts of other oppressors, to take the game at face value is do a disservice to the game's cultural and educational value.