I file this under bugs as I don't think it is as it should be.
Even in the new engine, outside ambient daylight will illuminate rooms and corridors inside buildings that have no windows. This is not only unrealistic but it does also impact the gameplay. If indoor illumination would be realistic the flashlight would be a much more important tool in daytime and the game would have far more atmosphere. Players/Zombies could hide in dark corners e.t.a.
This old bug was market fixed but I would like this to be reopened. As the fixes are not what I mean. I was looking for Sparse Voxel Octree Total Illumination support: this would give the following improvement https://youtu.be/Z-vuff9LDBk
I think it is important for gameplay, realism and overall visual quality that there is no light where there is no window inside of buildings during daytime.