Indentured servitude & equitable slavery in D&D

[Imp]

In my setting there are technologically advanced island city-states that have various forms of republican-to-oligarchic government and various forms of indentured servitude, which by centuries-long tradition lasts ten years. The more corrupt of these city-states have far more rapacious and unprincipled slavers guilds that seek to expand the scope of their institution, and take those defeated in combat as slaves, try to work their servants to death, etc: these are some of the black-as-night villains of the setting. The less corrupt of the states attach prestige to freeing servants before their time, provide loopholes that allow servants to reduce their terms, don't allow double indenturement, don't take servants in war, and have many legal protections for servants. In all cases the other, less advanced nations on the periphery universally look down on the practice, though they have feudal, caste, and other inequitable systems that organize their societies. Historically, it's because the indentured servitude system is a holdover from the days when an empire ruled over the majority of the campaign setting.