This mod has a rich history. It started here on the Ludeon Forums in 2018. It was created and maintained by Linq until 1.0, then updated by deathrat (1.1), Troopersmith1 (1.2, 1.3), and finally me (1.4, steam link pending).
A different mod which has similar functionality (patches a different method) is here.
If you're looking for a vanilla way to do this, you can go into storyteller settings. When you pick/change the difficulty, pick Custom. There's an option there for both enemy downed death % chance as well as colonist death on down %.
Feel free to use the GitHub repo and contact me regarding push requests, issues, feature requests or whatever you like.