Portland is not a good example of American urban planning gone awry. As it's probably one of the most sensible cities in the US.
And yes, unfortunately neighborhoods without sidewalks are all too common in American suburbs.
I suspect it's that way most places, and that's why you find so few without sidewalks. They do exist, though.
Mostly, but there's really 2 Portlands. Downtown & the eastside are dense, mostly flat, walkable and with excellent public transit. The west side is hilly, mostly suburban, and generally neither walking nor transit friendly.
Oddly enough, a lot of the neighborhoods weren't tightly packed. The lot sizes were decent and the house-to-house spacing was ample. I'm guessing it was more of an issue where, if the builders didn't have to spend money on sidewalks, they didn't.
It should be said that when some of these neighborhoods were laid out, most families had at most one car, and that car commonly smaller than some of the sports utes and minivans one sees today, leaving more space on the pavement for pedestrians.
The area was built up in the 1920s, and apparently at the time it was left to the homeowners on each block whether they wanted to pay for sidewalk construction. Most blocks paid for it but the homeowners on this street did not.
A sidewalk is not always needed to have a good walk.