I was considering this closed question very intently, and I found that I'm not at all fluent in the idea of modern slavery. Many philosophers have spoken on slavery. On this forum, someone has already asked of Plato's ideas of slavery. It is well-known that Hobbes and Locke considered slavery and the state of nature in their development of their conceptions of social contract theory.
But what really got me thinking was the fact that even some classic 'Utopian' philosophers, such as Thomas More, thought slavery had a place in their world. More, for example, thought that temporary slavery could be used as a punishment (rather, that it was used on his island of Utopia). Then, sometime within the following century, inalienable human rights began to take hold. And there are plenty of writings on that topic, and I don't care to name them.
But we got to the next step. People like Bentham argued that animals should have inalienable rights too. His famous quote involved asking not whether things talk or reason, but rather can they suffer? Although this hasn't taken hold, it made me wonder. Could a sentient machine suffer? Supposing we have a human-level artificial intelligence, what would it mean for it to suffer?