If you think so, then it likely heavily differs by country or other. Over here, if you showed people first picture, and ask them wheather they see on the side of the road: (a) footway, (b) cycleway, or (c) (illegal) parking; vast majority of them would answer (c) (illegal) parking.
That is because it is not usable as (a) surface for movement of pedestrians nor as (b) surface for movement of bicycles, but it is being used as (c).
Note that sidewalk/footway is just a footpath (surface desgnated for movement of pedestrians) that happens to be situated on the side of the road, and is often (but not always) somehow physically separated from it (kerb, tree line, bollards…)
I’d prefer sidewalk=illegal_parking (or more general sidewalk=not_usable) as it would lessen the most problematic possibility that someone who has not taken the time to read this discussion might naively add sidewalk=yes in addition to that sidewalk:use=illegal_parking, thus breaking everything for pedestrians)
A sidewalk is physical infrastructure, like a building.
I disagree. What would you call this thing on the left side of the road? sidewalk or cycleway?
I would most certainly call it a cycleway, as it is designated surface for movement of bicycles, and pedestrian traffic is not allowed. Thus, I would tag that highway=residential with additional cycleway:left=track + sidewalk=no . I’m very interested how would you tag it @osmuser63783 and @dieterdreist (and others)?
Yet, physically, it might have been footway/sidewalk instead (for example, if that image was taken from slightly different position, you might likely guess wrongly).
In other words, just because there is paved area separated by kerbs on the side of the road, does NOT automatically make it sidewalk. Sidewalk implies specific purpose (designated pedestrian traffic).