Apparently, a change from 1.13 to 1.13.1 was that mobs now require 2 blocks of air to spawn at all times, which pretty much breaks every creeper farm out there.

I've found this thread, but couldn't quite figure out whether it has been recognized as a bug or as a feature: MC-136226

If this is, indeed, a new feature, is Mojang aware that it breaks all creeper farms? Or even spiders, I'm guessing they won't spawn in 1 block tall 3x3 areas either.

Most importantly, does anyone know a way of working around this issue and still manage to make a creeper only farm? I need the gunpowder for my firework rockets...