From how I've understood it, Dishonored has always been a combination of different stealth feedback systems. Light and shadow as well as cone-based vision. This may be reversed, but shadows help from a distance and then when a guard comes close (within 30 yards, but I'm not sure how you will know that measurement in-game unless you keep some of the gamey cues or get really used to the visual ones) shadows aren't helpful. At that point, it's better to find physical items to obstruct the view cone or find a way to make the line of sight as narrow as possible since it can narrow greatly depending on positioning.
If I'm wrong about this or I was right and this has changed as you've said, I'd like to know too as it sounded like Harvey and Raf worked very hard at finding the right balance for these systems. It would be odd to change it now.
Edit: I should have read it first and not jumped to explanation since you probably were aware of all this already. The "not being able to destroy light sources" thing is new to me also frankly.