Even the modern world isn't as modern as we'd like to think...
Or as learned. Or as enlightened.
(again I'll point to the gender equality issue for easy comparison...
I don't find this an easy comparison at all. I find it a very strained one. I think an easy comparison to slavery in earlier times would be to the large number of actual (and not 'quasi') slaves and serfs (people without freedom of travel and without entitlement to the fruits of their own labor) living in the world today. In fact, by some counts, though the percentage of free people is trending upward, there are more slaves in the 21st century than there were in the 19th, and more slaves are being trafficed today than at the height of the atlantic slave trade. But, this topic tires me and is likely to provoke a flame war.
However, the modern world is actually far exceeded by most D&D worlds in the ability of one righteous character to make a difference.
I disagree.
A slave could conceivably learn magic to blast the guards....
We also seem to disagree over what makes someone a righteous character or how you might make a real difference.
Slavery is also predicated on ignorance...
No, it's not.
and I'd postulate that education is more widespread and prevalent in D&D worlds than it is in the modern third world.
If you didn't actually grow up in the modern third world, or haven't actually lived there, I wish you wouldn't base your assertions on mere sterotypes about it. While its not universally true, the education available in much of the third world is superior in quality (and more highly valued by those that recieve it) than what you probably got.
Certainly, almost anyone living in a sovereign kingdom in my world knows that there are outer planes where you go when you die and that divine magic comes from those planes and that anyone can access that divine magic and anyone can go to any of those planes upon death. Thus, it's hard to look on another person as being fundamentally different or inferior.
Interestingly, starting in the 1st century of the common era, there was a rather mystical cult - a Jewish heretical sect you might have heard of - that taught that no one was fundamentally different or inferior on the basis of social status, race, or gender and that in the afterlife a slave might concievably be expected to be honored greater than his master. And yet, despite these beliefs, they still lived with and even on occasion promoted slavery until well into the 19th century even after the cult had gained significant political and social power, quite arguably at least in part precisely because they believed that social station of a slave was temporary and even to a degree had a basic dignity which would be rewarded in the next life. These beliefs where held with such deep conviction that there was fundamentally no difference between the beliefs and knowledge for the purpose of how it compelled their behavior. This would I think strongly argue against the notion that knowledge of an afterlife would compel an adversion to slavery, a notion that is even more shaky when we consider that said afterlife in D&D might well include idealized slavery (as for example the Egyptians believed).
Granted, the general trajectory of the aforementioned cult's governance was toward extending freedom to a larger and larger sphere of people, but that there is no reason to believe that slavery is incompatible with the belief of the basic equality of people. For one thing, the Roman ethical codes that were in wide acceptance before this upstart Jewish inspired sect started up, also held that slavery was not enherently dishonorable, and like most of the ancient world often conferred great authority, wealth, and power on people who were technically slaves. In fact, many people of antiquity might have argued that being the slave to someone powerful was preferential in honor to being free but lowly. Certainly, you had a much better chance of getting three meals a day, and what is dignified about starving?
So in other words, there is more in heaven and earth than exists in your learning.
I think slavery would have been a great BoVD topic (with that hard-to-remove "mature" label).
Considering the level of immaturity on display in the BoVD, I would just assume that they didn't try to tackle any actually hard questions of ethics.