ZDoom

This feature could be really usefull!
It's pretty hard to achieve with dynamic light when you have multiple angled walls.

How is this be different from attenuated lighting, or just drawing a darker texture, or using a glow feature with black?

Later Edit: IDK what I was thinking when I said this. I think I just didn't have enough experience with glows yet. This is definitely a good idea.

A black or dark glow color is just that, an extremely dim glow. It can't be used to darken. Doom 64 lighting can't be used because it spans the height of the sector (including 3D floors) and so in cases involving certain pit setups would look awful. Drawing a texture would be limited to the resolution of the texture and won't actually affect the brightness of objects and geometry entering it. Subtractive lights can't be attenuated.

Even making the sector itself dark and lighting it with dynamic lights/glows will mean unwanted effects (glow gradient from no apparent light source, sudden darkness on walls facing away from dynamic lights, and using a large number of dynamic lights to even perform this effect in the first place).

Subtractive glowing flats would be super helpful for all the reasons Jaxxoon R listed.