In the earlier versions it was quite easy to get some white ground with some shade on it. The advantage was there wasn’t any border at the horizon visible.
You can try locking the exposure in the render window and then tweaking the brightness but depending on the scene, materials and lighting that may not be enough. Another approach would be to make a large planarsrf and then use the Bend command to bend up one end of it in the distance. With a large enough radius to the bend and sufficient distance from the model, you should be able to see the skylight shadows fade out.
