plug.kak usually supports latest stable release and development branch. I’ve decided not to support older versions of Kakoune because if done by branches, I have to backport many features using older methods, which I don’t remember, and because modules are thing now I would have to have separate code for versions that don’t have modules.
Some of my other plugins usually have tags for versions, but those don’t support specific Kakoune version, and are just plugin versions, though if you use old Kakoune build you might check out to older version of plugin, yet this may mean that some new features of the plugin will not be available.
plug.kak has ability to checkout to specific version/branch/tag, but doesn’t detect anything by itself. This is too smart to do. I think I could provide
stable switch, that will look for latest tag and checkout to it before loading plugin, though this is not what you want.