We are taught history as eras, each one giving way to the next in progression
That is false.
Isaiah was a Hudson's Bay man in Northwestern Ontario.
These ideas did not just go away. They just found other ways of existing.
Why include that exclusion?
Because the ideas remained even if the peculiar institution did not.
It's an interesting exercise. Drawing out your family tree and then overlaying it with historic events. Seeing how close and connected these things really are.