Oh wow, that's really cool. It's a shame it had to be closed, but hopefully the new work between you and Grondag can lead to a version of this in Forge, or at least in some form that's easily accessible by modders. I saw Grondag's mod when it was released, but I didn't really see the connection since those were custom blocks and didn't think that what was done there could be done dynamically to blocks on worldgen.
If BetterFoliage could make smooth leaves with that, and NoCubes got working on 1.12, both without coremods, that would be fantastic. The possibility of having Optifine-like features such as the better grass, better snow, connected glass, etc. but customizable would be a godsend to modders. I can think of so many possibilities for that.
Those very buggy screenshots shown in the PR makes me think that stuff like that could also be used to render directional block shadows, godrays, and stuff like that without shaders, even, if implemented. That would be brilliant.
I hope the performance issues can get solved and that some variant of the PR gets accepted, because clientside rendering tech is becoming a bigger and bigger part of many mods, and it seems to be an area somewhat lacking in Forge.