Exactly. And the even bigger point raising from it: if something is NOT Verifiable by mapper what it actually is, it should not be mapped by random tags which are not verifiable based on a pure guess.
The one in your first post is clearly a sidewalk,
Is it? “clearly”? How came then that this last (“black car”) picture is not “clear” to you then? It even shows much more of the surface than the first picture in the thread? If you are sure of the first picture, then you must also be doubly sure for the last one too. Yet, you seem to be unsure about the last picture.
you said as much (“built as one”).
Oh, so you too were born in that specific Zagreb neighbourhood several decades ago when there were much less much cars, and walked it often daily? Sorry I didn’t recognize you after all those years! 
joking aside: In other words, I do have local knowledge of that specific neighbourhood spanning several decades, so, against all chances, I am somewhat more familiar with that specific tiny part of the map more then the 99.999% of the OSM mappers (just like I am not so familiar with 99.999% of the rest of the planet).
So, what I can map here with source=local_knowledge there, you (most likely) can’t (not from the picture; you can visit and make friends with older locals and get to know a story over a cup of coffee, of course. The older generation there is generally a friendly bunch and likes to talk about times past!)
So, my source of information is not only that picture. From that picture alone, even I (living in that country) wouldn’t really know what it was initially before it became illegal parking. I might map it as disused:sidewalk=both, or as a disused:cycleway=both but simply because difference between “unusable sidewalk” and “unusable cycleway” is negligible. (as opposed to difference between “sidewalk” and “cycleway” which is huge).
This suggests another solution: Just don’t add a sidewalk tag
I actually agree with you here. This is why I actually did like that idea of adding just disused:sidewalk=both (without adding sidewalk=* tag), and probably why it was the most liked answer in the poll.
To conclude, how would you @osmuser63783 @Nadjita @dieterdreist (and everybody else) feel about:
highway=residential
disused:sidewalk=both
note=sidewalk is not usable for pedestrians as it is 24/7 full of illegally parked cars
one might even invent (and add to that list) suggested tag like sidewalk:both:obstacle=parked_cars or similar, if one thinks that machine-readable reason (in addition to human-readable reason in a note) is important addition for determining the reason why the sidewalk is not usable.
It specifically doesn’t add any sidewalk=* attribute, to keep common ground, and avoid contentious choices. Would that work for you?
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