It is curious to think about how the use of the words "horizontal" and "vertical" in photography and other print media is slowly fading from use in favor of the terms "landscape" and "portrait". The question isn’t, "When did it happen?" but "Why?" Regarding the when, there was not a single day, but rather a slow progression over the past decade or so partly as manufacturers of computer printers started using the terms in their printer software. Now it has changed our common vernacular.
But the question remains, why? Perhaps it was because they needed a symbol that would correspond with the words horizontal and vertical and an icon of a person and some mountains made sense. Unfortunately, their need for an expedient symbol for their printers has not only changed our vernacular, but it has made us associate portraits with verticals and landscapes with horizontals. In other words, it has reinforced our visual box. As if most of us didn’t have enough trouble getting outside of our own boxes, a new one has been created for us. Use whichever terminology you prefer, just know that a portrait can be horizontal and a landscape photo can be vertical.