The first 'S' word is Sex. As we know from Phil and Dixie, they avoided talking about it while trying to get to talk about it.
Slavery is the other word.
Slavery was common in the way back.
For some categories of slave, it wasn't even a bad deal. A Greek tutor was a good deal for a slave in Rome.
But medieval slaves are laborers, not skilled experts.
Despite the medieval trappings, most D&D campaigns don't have slaves.
I think it's from the generally egalitarian nature of the players.
Most of the fantasy games I've played didn't have slaves and didn't discriminate against women.
The one campaign were slaves were prominent had my character leading a revolution to end slavery. That's the only time it stood out.
OG AD&D has a couple mentions of slavery in the Dungeon Master's Guide. Admitting it exists is a pretty big deal.
The World of Greyhawk has slaves. It also has some powerful abolitionists.
AD&D 2e has an admission that slavery could exist, but only in the context of humans being weak and other races being strong.
The evil nations of Forgotten Realms have slaves and a slave trade.
AD&D 4e mentions there's enslavement and suggests a campaign where the players free slaves and end tyranny and mentions being an escaped slave as a character background.
AD&D 5e mentions slavery as a tool used by the villain in a villain's methods random table.
GURPS: Banestorm, being a world setting rather than a game, has a chapter on slavery as does the 3e book of the same setting GURPS: Fantasy. (4e's Fantasy is for making your own Fantasy world and it explains all the tropes and let's you DIY a world from them).
With all the mention of slavery in the rules, why didn't more of us have slaves in our games?
Again, I think it's just our American egalitarianism being applied to a social order that was decidedly NON-egalitarian in nature. Feudalism isn't, generally, compatible with how Americans see themselves.
All that romantic liberty, freedom and equality (distinct from the French "Liberté, 'egalité, fraternité) seeps into most everyone's game and makes an almost unconscious decision to not go historical feudalism with it's attendant tyrannies and rigid social order.
The players don't like it when you put such things into the game. They never take any social status that makes them noble and when the restrictions of being a commoner hit them, they get peeved.
Enforcing the sumptuary laws on them was good for a laugh. But they really get umpty when they get told they can't buy a common everyday weapon, like a sword.
That first came up when I played a Togugawa Japan campaign in GURPS. A commoner might own a wakazashi with a permit; but only Samurai could own and carry a katana. The VERY rigid social structures confused the players and they didn't work to understand them. Not that I understood them well, but I was working at it.
Japan is interesting here because, while the structure was rigid, it was possible to break through it and move up. Not common, but possible. Toyotomi Hideyoshi being an excellent example.
In Europe it was much harder to break out of your lane. At least until the plagues...