It’s a git patch file, however his code needs some tweaking to work on 4.26. It’s not too much effort, just go through his patch file, find all the source files it modifies and make the changes manually. I tested his and it took around 25 minutes to get going. I actually recently wrote my own retroreflective shader model, which is very much a WIP
It works differently from william.sch’s as instead of just adding a second specular term I used a second layer similar to clear coat and lerp the specular reflections, allowing it to be masked, with full support for rect lighting. I’ll be posting a git when I finish it up with the patch file for 4.25/4.26. Just note that mine is a little more complex, adding 3 new inputs to the material graph output node & material attributes node. I’ll update this when I post it