If your mod is successful then you can always backport in the future. The best way to code a mod is to write it for the latest version and do your best to make it is future proof as possible by abstracting what you can and always using the most up to date way of doing stuff.
Fix this, you really need to be able to see the vanilla code for modding. If you have trouble with it ask for help.
Look at the horse code. It’s probably something like a timer in the entity + setting rotations in the Renderer based on that timer.
Read this: https://cadiboo.github.io/tutorials/1.15.1/forge/1.4-proxies
You’re better off extending the Arrow and adding the spin in your renderer (similar time the arrow shake) as it already sticks into stuff