Toggle combat versions

There should be a togglable option, before creating a world, to switch between 1.8 and 1.9 pvp (maybe even combat test 8c) or other future potential combat versions. This would work for both Java and bedrock. Once a combat version is selected and the world is created, the combat toggle will be disabled. Bringing back 1.8 will allow servers like hypixel to update to the latest version of minecraft, While still keeping the old combat. 

Potential idea: Being able to toggle certain features of each combat version, example: Having shields in 1.8 combat, or using 1.8 rods in 1.16 combat.