It's definitely a decision you have to think about carefully.
Personally, I think including slaves, and depicting slavery harshly, can make a game more intellectually interesting.
But it's a difficult balancing act you can't commit to halfheartedly:
- Slavery needs to be depicted...
- ...but not glorified. You don't want players to play the "Dark Side Jedi" and gleefully participate ("I'm only roleplaying!/It's only a game!").
- ...but the game itself still needs to be fun and enjoyable, and not too somber.
- The cruelties need to be explored and demonstrated...
- ...but not glorified. You don't want some people to enjoy the portrayal of domination and subjugation feeding their fetishes.
- ...but not minimized. Slavery, while commonplace in that time period (and today as well), shouldn't feel 'normal' to the players.
You have to balance being preachy and ham-fisted, with not indulging people's fetishes, not accidentally teaching that slavery is profitable (despite that being the reality), but also not minimizing or treating it casually.
I remember reading about a teacher teaching a class about the African slave route, and had students load up their boats with "humans" and give each person a name and family members on the crew. Then they began crossing the Atlantic, and (by rolling dice), named slaves were thrown overboard, died from starvation, died from disease, etc... etc... with the end result that few made it to the other side.
I think the easiest way to do this safely, is to make players observers and not engages with the slavery, or to reveal after the fact that actions they took furthered some horrendous crime.
In a board game that you replay, I think the deaths and abuses become too commonplace in players' minds, so I'd save those kinds of explorations to single-player games that players usually only play through once or twice.
If you do wish to add slavery, maybe slave traders are a non-player dynamic that's occurring in the world that interact with players but that the players never get an opportunity to profit from. If the game already has the player have crews, maybe name each crew (players draw cards for their crew members) and the crews have different stats and maybe can be upgraded, so players attach to the players, and suppose some of the players' own crew gets randomly captured by slavers, and they find out (three or four turns later) what that person's fate was (what kind of death, what kind of slavery, in what location, from what slave ship), and maybe players have the opportunity at capturing slave ships, freeing the slaves (and risk sinking the ship and drowning the hundreds of slaves chained up in the hull). Or something.