I agree that pay stability is new, but that doesn’t mean they were in a contracting relationship. For most of recorded history, most people were engaged in some form of subsistence food production. Many were enslaved. Generally speaking, neither of those economic models are that close to “contracting.”

Tenant farming gets closer (although not really). Itinerant labor probably the closest, but I don’t think they made up much of the population in societies I know about. Skilled craftsman could also count, although they look more like a generalized small business than a “contractor” as at least I think about it.