There’s a number of factors that would need to be taken into account.
If an object moves, then not only is the area around the object considered changed, but also the area where the shadow lands.
If there is a reflecting or refracting object, then any area that shows the changed object in a reflection or through a refraction would have to be considered changed
If you add GI to the scene and use physically-based specular behavior (replacing it with true reflection of light), then if an object is changed it could affect the lighting in large areas of , if not the entire scene and also affect what is reflected in glossy materials or refracted behind glass, to only change the object when it comes to location and visibility may not even be practical and the recoloring and rapid re-calibration of lighting and shadows in the scene due to material changes may complicate the code a bit too much if all cases were to be covered (not to mention require a ton of coding work)
In other words, I can’t see this being any sort of priority for Brecht until Cycles’ feature list not only includes everything BI can do (that includes render passes, SSS, and volumetrics), but also after other, more practical optimizations is done (implementing faster raytree methods like QBVH and optimizing the shaders and the sampling algorithms)