Minecraft wiki entry for Experimental_Gameplay, associated text: Experimental Gameplay is found in the world options when creating or editing a world. When turning the option on in an existing world, a copy of the world will be created automatically. Turning the option on does not disable achievements‌[until BE 1.18.30], however it can't be disabled after the world is created. Once the experimental world is created, it cannot be turned off again. Worlds with the experimental on will have an "Experimental" tag right next to the game mode name in the world menu.