I posted this same question on the Godot Q&A, but I didn't have a decisive answer; since Godot Facebook group is getting crowded (and somewhat messy too), I thought on posting here.
Thing is, from my understanding, the only way to apply Shaders on a 3D environment is by the use of materials; I'm completely clueless on how to apply this sort of Shader as other implementations I found online. I would have to get information on 3D distances and stuff, and I didn't found any documentation regarding that on the Godot Shader page. Is there a way to do it?
BTW, I tried using the WorldEnvironment node to no success. Maybe the way I set it up is wrong, but another user reported it looked broken (https://godotengine.org/qa/10569/how-to-create-a-fog-effect-using-shaders). The project that I want to apply this in is a 3D Dungeon Crawler demo, you can check it out on my GitHub: https://github.com/henriquelalves/simple_dungeon_demo.