when serfdom died out in England You’re equating serfdom with slavery when my point is separating slavery by name excludes such classes. India and China for example where very stratified societies, but slavery was effectively a separate and less common institution. https://en.wikipedia.org/wiki/Slavery_in_China

https://en.wikipedia.org/wiki/Slavery_in_India

Anyway, by relatively uncommon I mean compared to the US concept of slavery where they made up a huge chunk of the southern workforce and could be sold between individuals.