While skimming through spigot’s rather vast collection of plugins, I came across a gem that appears to alter 1.9+ combat to be like 1.8 and back combat.
I just thought it would be something interesting to port to sponge.
https://www.spigotmc.org/resources/oldcombatmechanics-disable-1-9-hit-cooldown.19510/
I may port it myself later. I’m just posting it here for record keeping and if someone else want to take a stab at it.
Here is the github for it that I found. https://github.com/gvlfm78/BukkitOldCombatMechanics