All of civilization is based on slavery, going back roughly 10,000 years. Stanley Diamond summed it up in the sentence “Civilization began with conquest abroad and repression at home.” Cities wouldn’t exist, since the first cities were created as resource centers for the rulers to store their larders and distribute stuff to their slaves/serfs/whatever. Instead, humans would have stuck to smaller camps and villages. Technological complexity would likely have reached some sort of cost/benefit plateau resembling something like what we think of as neolithic or early “Bronze Age”, with some exceptions. Without slaves being forced into monocropping, free individuals would probably have continued their sustainable lifestyles of hunting, gathering, gardening, permaculture, etc, and so the formerly fertile areas of the Middle East and the Mediterranean would still be fertile and thickly forested; the thick forest canopy that once covered present-day Iraq, scarcely letting light hit the ground, might still be there. It would also mean improved overall health; archaeological and historical records show consistently that the adoptions of monocrop agriculture creates reduced overall health within communities, which are especially apparent in bone and tooth related diseases and drastically reduced lifespans, as well as endemic diseases such as cancer, diabetes, and heart disease. Without slavery, economic systems would have remained largely gift economies augmented with limited trade. Women in such societies always fair better than their agricultural counterparts, having equal and sometimes greater political power than men, longstanding and effective family planning traditions, and freedom to divorce errant partners.
I’m sure I can think of more, since this has one of my keenest interests as an anthropologist. Maybe more to come.