Hiya. I'm one of the devs for Nell's Window. Glad you enjoyed it and thanks for the feedback! :D
I made the pixel shader. Though for annoying URP-to-Web build reasons it wasn't on half the time during testing, hense why some things are difficult to read. As it was tested on builds without the shader.
Regardless, I'm happy to show you the shader graph I made, if it helps you out. (un)Fortunately Unity has a built in dither node which I'm not sure Godot has (I assume it's just a form of Ordered Dithering)? Which probably makes this look a little "step 2. Draw the rest of the owl":
There was also a feature to have a gradient colour mapping if we wanted to go full Return of the Obra Dinn on the art style. Though we never used it.
(I've been meaning to remake it in Godot as well lol)
Hope it helps!