A sidebar could be a content warning. It’s not my job coming up with a full check list, but I know that a professional sensitivity reader could give advice how to present it to today’s audience demographics.
It’s easy to be uncharitable towards the concept of sensitivity readers. I used to roll my eyes at them. After reading up on what some professional readers have given input on, I’m a believer they offer real value.
All of us could use reminders on how to not be “a tool”. No one has the lived experience of everyone who might play the setting. It shouldn’t be the cause of resentment when the guidance appears in new products just because it didn’t cross the author’s minds in previous editions.
Ive never been uncharitable to any sensitivity readers.
I suppose on reflection actually, that for a given modern audience, it might be worth contextualising the slavery. Obviously, given recent events over the past few years and the relatively recent historical baggage of black slavery specifically, it might serve as a reminder of the kinds of concepts of classical era slavery, indentured servitude etc. Thinking of the children in the school I teach in, the term slavery would just just conjure images of black specific slave trade from their history classes.
But this should be acting as a contextual primer (it doesn’t change anything about the setting) rather than a patronising sidebar a la Ravenloft (imo).
A content warning would be a bit late at that point in the book. Wotc could just save themselves plenty of time and just stick warning disclaimers on the backs of their book as tv shows do, “warning, contains themes of slavery, cannibalism, player discretion is advised” and be done with it.