So, YOU don't see any issue with it. The Idea of invading a nation and taking those who don't agree with a specific ideology (that undead is a better state of being than alive - hence the lich) and taking those who fight against it and putting them into work camps until they die is, pretty much, most people's idea of evil. The fact that someone can make a parallel to work camps in WWII and that killing people who don't believe in your ideas for a 'better world' isn't an impossible idea.
It's there even if you have to dig for it. Some people might see that parallel because they've experienced it or have family who have. I could see it as a sincere complaint and you wouldn't be able to just write it off as 'overly sensitive complaining'. People on this board have legitimate complaints about specific content and others have dismissed their complaints. It's a thing, even when it doesn't fit your view of what warrants a complaint.
So, the question is, how do you publish anything when you risk having to pull your books off the shelf? Why pull the books off the shelf at all? This has been raised before about books outside of D&D: Tom Sawyer, To Kill a Mockingbird, Of Mice & Men. People asked that those books be pulled from libraries and schools. Is it right to do that? Is that how people want their society to function? Or maybe there should be a forward added to new prints of those books to put them into context? Chances are, the rules for what is PC today won't be PC tomorrow, so do we just get rid of all the content from the past or do we educate people about it and continue to release new, up to date material?
It would not be hard to edit a copy of the PDF of Oriental Adventures and add a forward and to add addendum in the book. They release erratas all the time to PDFs that do exactly that.
I agree, release better material but you can't crucify every artist for their material or prevent people from reading it if it eventually suddenly becomes outdated.