I am replying to a comment made in another topic because I think it is more on-topic here
Well, there is more than enough evidence that the Buddha opposed cruelty. He rarely - if ever - went to the extent of getting political about it, I think he gave general spiritual advice and let the people figure out for themselves how that spiritual advice should be applied in non-monastic daily life.
So I don’t think it would be even controversial to say that the Buddha would not praise the kind of extremely abusive slavery that is for example portrayed in the movie 12 Years A Slave, given that he spoke clearly against cruelty. But even slavery comes in all shapes an forms. One can surely imagine a situation where someone is socially recognized as a “slave” yet is not subject to cruelty, and this is a scenario where it becomes a bit more difficult to have a clear answer.
I think it is quite clear that the suttas portray “imprisonment” (bandhana) as a bad thing. So it would not be too far fetched to consider that he would regard depriving a person of their freedom as unwholesome. If this line of thinking is correct, it could still allow for “slaves” but they would have to be for “free” all intents and purposes regarding the “master” (even if they still regarded as “slaves” by the rest of society).
Also, I think it could useful to look at Ashoka’s edicts if we want to widen the scope of inquiry, as Ashoka is often regarded as a model of Buddhist leader.
Edict 5:
Now, in times past (officers) called Mahamatras of morality did not exist before… They are occupied with servants and masters, with Brahmanas and Ibhiyas, with the destitute; (and) with the aged, for the welfare and happiness of those who are devoted to morality, (and) in releasing (them) from the fetters (of worldly life). They are occupied in supporting prisoners (with money), in causing (their) fetters to be taken off, and in setting (them) free, if one has children, or is bewitched, or aged, respectively. They are occupied everywhere, here and in all the outlying towns, in the harems of our brothers, of (our) sisters, and (of) whatever other relatives (of ours there are). These Mahamatras of morality are occupied everywhere in my dominions with those who are devoted to morality, (in order to ascertain) whether one is eager for morality or properly devoted to charity.
Edict 9 & 11:
Herein the following (are comprised), (viz.) proper courtesy to slaves and servants, reverence to elders, gentleness to animals, (and) liberality to Sramanas and Brahmanas; these and other such (virtues) are called the practice of morality. Therefore a father, or a son, or a brother, or a master, (or) a friend or an acquaintance, or even a (mere) neighbour ought to say : "This is meritorious. This practice should be observed until the (desired) object is attained, (thinking): “I shall observe this”.
Also somewhat relevant is Edict 1:
All men are my children. As on behalf of (my own) children I desire that they may be provided with complete welfare and happiness in this world and in the other world, the same I desire also on behalf of [all] men.And you do not learn ? how far this (my) object reaches. Some single person only learns this, (and) even he (only) a portion, (but) not the whole. Now you must pay attention to this, although you are well provided for. It happens in the administration (of justice) that a single person suffers either imprisonment or harsh treatment. In this case (an order) cancelling the imprisonment is (obtained) by him accidentally, while [many] other people continue to suffer. In this case you must strive to deal (with all of them) impartially. But one fails to act (thus) on account of the following dispositions: envy, anger, cruelty, hurry, want of practice, laziness, (and) fatigue. (You) must strive for this, that these dispositions may not arise to you. And the root of all this is the absence of anger and the avoidance of hurry. He who is fatigued in the administration (of justice), will not rise; but one ought to move, to walk, and to advance.