What are you talking about? Practically every society engaged in slavery until the modern period when societies eventually outlawed the practice. The US fought a war over it, in which the northern liberal humanists were opposed to it. Radical pseudo-liberals, as in the southern libertarians who wanted laissez-faire capitalism championed slavery as an issue of private property rights. But that's semantics. Libertarianism in that sense was always a bogus, unsustainable, antisocial spinoff of liberal secular humanism that reduces to the conservative's social Darwinism, as I've argued elsewhere: