I created an iron golem farm in creative and discovered half slabs restricted their spawning area. I tried to transfer this to survival but the golems were able to spawn on the half slabs. The wiki indicates that the intended behavior is that the golems should be able to spawn on the half slabs. Attached picture of my creative farm, the golems will only spawn in the upper left area where I have removed a set of three slabs, the rest of the ground and any spawnable area is covered in half slabs of various types. The creative world is a super flat redstone ready world.