Remember YGMV, just because it's in the rule book does not mean you need to use it. Your characters tribe might be strongly against slavery, hell, why not make a senario out of freeing slaves from a "less civilized" tribe. But as you say, slaves in one form or another has existed for most of human history and and ignoring that completely in the rules would, at least to me, be dishonest to the setting. In my game, slaves exist but are never in the forefront, and if a player or NPC is captured and the tribe fails to provide a ransom that character might very well end up in slavery.