In the new snapshot 20w17a it was made that you can change gamerules when creating a new world, but you can do this even if cheats are not enables with gamerules where it doesn’t make sense that you can and on bedrock edition and console edition you are also unable to do this. 

How to reproduce:

1. Go to Create New World button in menu.
2. Go to gamerules.
3. Try to change a gamerule like for example spawn mobs.

You can do this without any issue even if you try to create a new world.