When I go to my private server, it says its not compatible. And when I playing on it, the mod doesn't work. It only happens with this mod.

Hello! I have made a plugin for Paper 1.16.5+ out of experiment. You're welcome to test if it works.

https://github.com/Brawaru/DamageTiltHelper

In the Spigot mappings this value is already calculated and has the name ap (A public field of the EntityLiving class, at least in v1_16_R2, this name is probably unstable). It can be sent to the player every PlayerVelocityEvent via the messaging channel, but I suspect issues similar to #5 will happen, so this needs some testing. (e.g. defer the damage Event beforehand)

Edit: yep, you can get the EntityLiving of a player with

Player player = event.getPlayer();
EntityLiving player_entity = ((CraftPlayer) player).getHandle();

I would really like to see a server-side only version of this for Bukkit/Spigot!
Also, is there some sort of history thing for Forge methods? I'm on 1.6.4 and I'd like to know if this could work.

The mod unfortunately isn't client only, it has to be on the server as well.
If you put it on the server, the clients will need the mod too.

I was trying to do that, unfortunately they dont have this mod on the list mods for the server.

Sorry to bring the old issue up, but would it be possible to write Bukkit plugin that would calculate and send attackedAtYaw via messaging channel?