This simple mod was made for people who want the old healing rate. That's it.
I made this, because I talked through it in an episode of 'Experimental Cataclysm' and figured I'd do the work for the viewer, because we all know how lazy people are.
Simply download the zip folder of the repository. Extract it. Place the folder in your cdda\data\mods folder and it should appear in your World Creation screen under 'balance'. I will not update this and do not use it personally. Do with it what you will.
I'd encourage you, though, to play with the default heal rate, because the game is going to be balanced around that. Also because it's not that big of a deal.