In previous versions, Iron Golems would not spawn on carpets. I know this because this specific farm design worked and the villagers would spawn the golems on the provided platform, not the building (as shown in the image below). If this is not considered a bug I believe it should be due to the fact that they should follow some of the same spawning rules as regular mobs do.

Steps to Reproduce:
1. Make a villager frightened, ie. zombie
2. Give them no place to spawn a golem nearby besides a carpeted platform within a few blocks
3. As you will observe in 1.14.4 Golems are able to be spawned by villagers on carpets