I was on a creative single player world on which I had customized the terrain. I wanted to test if doMobSpawn prevented passive mobs from spawning because I couldn't get a clear answer on the internet. After creating the world for the first time I let it load in and then set doMobSpawn to false. I flew quite a while in one direction and saw cows, sheep, pigs, horses, rabbits, donkeys, and squids. I then set the time to night, to make sure it was working, and immediately hostile mobs started spawning. I double checked that doMobSpawn was false and it was. To make sure I was using it correctly I set the game difficulty to peaceful so the hostile mobs would despawn, set doMobSpawn to true, and then set the game difficulty to hard. Hostile mobs started spawning again immediately. I'm going to attach a screenshot to show you what I mean.