No one wants to acknowledge slavery exists/exsisted except as benefits a certain demographic. People would rather put thier heads into the sand and ignore it exsisted and in some places still exsists. However, you can worship a world destroying evil being and have no ramifications. Go fig.
It's up to the DM to know his own limitations. If he doesn't want the hassle of a half-Aardvark/ half-Tarrasque Ranger/Artificer then he should tell his players. It's the same for any game i.e M&M, GURPS, Pathfinder, etc... Basic DnD is PHB/DMG/MM. Everything builds on that foundation...