with Next Major Update enabled, you can sneak lower, but you can't actually go in 1.5 block tall spaces, which I'm certain is what's intended. Also, block selection is offset upwards from the crosshair and camera (when mining a block, often the block that's being mined is above or behind the block that the crosshair is within). In the attached screenshot I am looking at the grass block with my crosshair but the selection area is around the slab.