A search in the Suggestions forum for "combat gamerule" lists ten different suggestions for this, and that's not even counting any suggestions which have nothing to do with gamerules. Over and over and over, the same points have been brought up:
1. The point of the combat update was to add a new mechanic, why should Mojang add a feature to disable an intentional feature meant to balance combat?
2. On this forum, you will find more people that like the update than people that dislike it.
3. Game rules themselves are meant to assist in mapmaking, not for people to pick and choose what kind of game they want to play. Could you imagine if real world laws and jurisdictions worked like this?
4. Games change over time, and should be allowed to do so. Holding onto old versions is pointless because players can play older versions of the game anyway and it means Mojang can't improve their game when broken mechanics that people use have to be kept in because people depend on them like their lives are at stake.
5. This splits 1.9 haters and lovers even more, not even less.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.
Mojang has said time and time again, that they WILL NOT be adding a gamerule or reverting the combat changes, Seeing this same suggestion over and over and over again is just a waste of your valuable time being alive....