10:37 PM, Thursday March 26th 2020
The horizon line would stay relatively constant assuming you only ascent/descent without rotating your head/eyes/camera. This assumption only starts breaking once you start getting into outer space or if the real horizon line is becomes hidden by buildings/terrain.
So what was the notes talking about? Whenever the viewer in a drawing is raised it is also assumed the viewer will rotate their head/eyes/camera and start looking down on the scene. Similarly when the viewer is lowered down closer to the ground it is assumed the viewer rotate their head/eyes/camera and look up at the scene. This is to keep the focus on the same target as the viewer is moved around.
It is this up/down rotation that causes the horizon to move. Rotating the view downwards makes the horizon go up in the drawing, at the same time the viewer is raised providing a birds-eye view of the scene. Rotating the view upwards makes the horizon go down and at the same time the viewer is lowered to gets a worms-eye view of the scene.
Of course nothing stops you from placing the viewer high up and still look upwards, on which case you'll get a nice view of the sky or ceiling. Or placing the viewer low to the ground and looking downwards, which will provide a up close view of the ground. These cases don't get as much use though.