In this case, I think it comes down to how to abstractify a concept without trivialising it. Both abstraction and trivialisation rely upon reducing an issue down to its core ideas and how that is presented. In this setting, trivialisation could be loosely defined as perceiving an abstraction to focus on the incorrect core aspects of a concept (or at least losing a vital aspect in the abstraction).
With Endeavour, personally I think it works relatively well as the abstraction of slavery is a similar level of abstraction of the rest of the colonialisation mechanics. This isn’t a game with heavy mechanics into the minutiae of colonisation, with an over simplified side of slavery. The whole game is heavily abstract and the use of slavery sits within the game on the same plane. It recognises slavery happened (with benefits to the slaver nation), and it recognises slavery ended.
I can’t see how a game can comment much beyond that without imparting modern sensibilities. It’s fairly matter of fact in its presentation, but as a distant viewpoint it works in basic terms. There’s never going to be a nuanced argument in this level of abstraction.
I think it’s quite brave of the game to acknowledge the economic advantage of slavery to get an edge over rival nations. It’s such a taboo in modern day discussion on the subject, which rightly centres around the abject immorality. The intensity of slavery in this respect is neutered by the distant abstraction of the game.
How it reduces down to a Y/N option is a more nuanced debate than I have the education to participate in. With historical subjects I think it’s important to enable the viewpoint of that time, without revising history to fit with modern views or encouraging people to impart modern morality upon the issue. We learn nothing by presuming everything that has happened since was an inevitability. But that’s impossible within a game unless it was accompanied by a full companion book. So I’m at an impasse on that one! I do respect how the game allows all or none of the players to enter into slavery.
Compare that to Archipelago, where Slavery is one card that one player can take. It really sets an odd tone. Only one nation can commit slavery? The other players see that player as the bad guy as if the rest of colonialism was a party? I’ve played so many games where that one card completely changes the room - people often actively avoid taking that card despite its mechanical benefits. It’s interesting from a social standpoint, but from a game design is so odd.
Endeavour still runs into the same issues of how the audience may respond to the abstraction in an inappropriately light tone, but I don’t think it’s up to the medium to take responsibility for the behaviour of its audience.

