No, it is not possible to leave without the glider. As mentioned by multiple users in this thread, the glider is not necessary to descend, however, the fog that prevents you from leaving will not dissipate until you obtain the glider. That is, once you've already obtained the glider, it seems that you can descend the plateau without it, but you can't avoid actually getting it.
The real problem is not physically leaving the Plateau, it's the fog that makes you despawn. The paraglider removes the fog, this video doesn't leave the Plateau (the map is available both at launch and landing)
Have you even played the game? You cannot leave the plateau without completing the first four shrines and obtaining the glider. Its not the height of the plateau that stops you leaving, its the invisible barrier of fog surrounding it.
The video that spawned that particular discussion is here, and even there, the person who posted the video commented that this does not negate the need for the glider:
Nah. You need the glider to despawn the void out area around the plateau. If you could get past that you can skip it.
