The Norseman Rule is named for a player from the defunct SDNW2 STGOD who opted to portray a "modern day" nation that still had chattel slavery and which dogmatically defended their right to maintain it. Though he provided one decent storyline through that setting (albeit with material that invoked righteous fury in many others), ultimately his fate was to irritate his fellow players until finally Shep opted to deal with him permanently by launching a massive biological agent attack on him, resulting in what was called the Pathogen War in SDNW2 and which nearly killed the then-2 month old STGOD. When the dust settled Shep and Norseman had seen their countries destroyed and the rest of the world was reeling, the players feeling it necessary to inflict a crippling economic recession (depression in some cases) as a result, and lots of stuff generally getting disrupted.
As a result, SDN World participants no longer wish to put up with players generating states where open chattel slavery is legal (including alternate "forms" that are clearly slavery when you file off the title). Therefore any player who makes slavery a legal aspect of their state is opening themselves up to the invocation of the Norseman Rule, ultimately resulting in the offender being curb-stomped by a Coalition Clusterfuck put together by the others (exceptions may happen, likely directly relative to how annoying they are about it).
IOW, if you wish to pursue slavery as existing in your state as something other than a nasty thing that occasionally happens thanks to the criminal underworld and is punished whenever outed, be forewarned that you may find your state getting crushed by the rest of the player base in an ass-beating of unholy proportions.