A short article titled "Voxel Landscapes 2D and 3D By Scout/C-Lous" described most of the tricks, but I can't find the text anywhere. A similar article by the same author on another topic I found here https://github.com/ggnkua/Atari_ST_Sources/blob/master/Docs/...
Searching for "voxel" in https://github.com/nickelsworth/swag/ also finds some very similar tiny Turbo Pascal programs.