In 1.12.2, it was possible for Iron Golems to spawn within half slabs, as long as the block they stood on was a solid, opaque block. This allowed for very efficient spawning platforms in advanced iron farms, utilising many slabs on top of each other so that the maximum amount of spawning spaces for iron golems could be achieved.
However, in 18w30b (and possibly the initial 1.13 release) Iron Golems need to have completely cleared space above the block they stand on to be spawned. This greatly reduces the efficiency of these spawning platforms that are the core of many large iron farms.

Though it is bad to allow golems to spawn within full blocks (as was the case in a recent bug), they should still be able to spawn within half slabs, like they have been in 1.12 and previous versions.