When building a creeper farm in the new 1.11.0 update, I found that mobs other than creepers and Spiders (i.e. zombies and skeletons) will spawn in 1.8 high spaces. When building a platform with a two block gap and trapdoors on the ceiling  in order to test whether or not the fix listed in the 1.11 patch notes was accurate, I discovered that groups of zombies and skeletons would also spawn beneath the trapdoors.