This kind of visualization can be really useful for impaired people. I can guess that street steepness is an important concern for people in wheelchair, a few degrees more and it become impossible to move. And even if topographic contours may be considered as the gold standard, it is often barely readable for big cities: I consider myself good as reading map, but being able to see steepness of street in cities is hard and requires a lot of mental work. Providing this kind of easily readable map for tourists would then be useful for mentally impaired people, which is even more useful as mental and physical impairments frequently come together. I would go further and compare this with metro maps: if they are not represented on traditional topographic maps it is for a reason: readability. And the proposed method offers a good readability of steepness for people needing this information.

So yes, having this kind information available is nothing new. But no, it's not reinventing the wheel as other people may have suggested here.