I am not saying we need to wallow in the horrors of slavery, colonialism, terrorism, fascism, etc... But I do think that we do not want to erase those things from our game worlds, because they give us the opportunity to create heroes that feel more real -- because in the popular imagination at least, it is rising above injustice (in all its forms) that makes heroes.
All this "we" and "our", as if we had to all do the same thing.
Indeed, we can turn this around a bit, and note a trope I've often seen discussed. When many writers of fiction want to show a character is "tough", they do so by introducing trauma into their backstory. With female characters ,the go-to trauma seems to be sexual assault.
Because, yes, sexual assault is a real thing. Even a common one, in the real world. But it is also
super easy to use. Cheap, even, if not handled with sensitivity and some understanding of the realities, which are often not in evidence. When it becomes common, but handled poorly, the result is unsatisfying, even inslulting to real-world survivors of the trauma.
I have witnessed my wife pick up a book, and nearly instantly physically toss a book aside, saying, "Welp, by page three, the main character has been raped. I'm done."
So, using these controversial, adult, or historical issues can be fraught - they give you the opportunity to make characters that feel more real... at the cost of not actually understanding the reality.
Do you scrub your world of slavery and other historical crimes? If so, how do you encourage heroism?
This kind of weird skew of the world sometimes happens - if you like a thing, or find it valuable, it suddenly becomes
required. But it isn't like historical injustice is the only valid source of conflict!
If a bunch of PCs goes out after a dragon that's been ravaging the countryside, or to lift everlasting night that has draped the land in everlasting winter, there's no "historical crime" (as I think you are using the term) involved. Heroism is about taking personal risk for the benefit of others - that can be done fighting slavery, or rescuing someone from drownings in raging rapids.
If not, what do you do to mitigate the real potential discomfort such subjects can cause?
Have an adult discussion with the players about what bothers them, and asking about what is fun, what is tolerable, and what isn't, and how they'd like it handled.