Is actually pretty simple to add a version switcher to ZeroDay (Would love to see that!)
Take for example this mod right here:
https://github.com/ViaVersion/ViaFabric
Is possible to integrate it into a client.
(I know Java is a hard language to code on [Personal Experience])

Enhaced2 Nef said that this may be added in the future but isn't currently planned as of last I remember. I seem to recall him saying it would be added but this may have been satire so don't get your hopes up.