Guys, I've just realized that you can avoid this annoying "clouds(hdri)+volume haze horizon rectangle bug" just by adding refractive surface, for some reason this rectangle is just not visible through refraction, so I come up with creating a huge non-reflective glass plane (see the attachment). I don't know for what reason do you need this :D but maybe devs could implement this fake solution (checkbox "create huge glass plane on the horizon line) until they come up with something better.