It seems as though the world spawns passive animals when creating new chucks; ignoring the fact that DoMobspawning gamerule is set to false. It doesn't appear that they spawn on already generated chucks though.
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I was unable to find a clear answer to that question on the internet, but to the best of my knowledge, it's for both hostile and passive mobs.
You have the wrong capitalization on the gamerule.
I wasn't sure what the capitalization was when i typed it in the report, but I used tab in game, so I know it wasn't wrong, but good idea.
DoMobSpawning is only for mean mobs right?