City planners in Halifax aggressively demolished the poorest and highest density areas in the 1950's and 60's, and that correlates with taller attached buildings. Many blocks like this were torn down:
Source
Source
Most stuff in the bottom half of this image (lots of row housing still survives in the upper half):
Those detached South End houses with brick walls were originally for much wealthier owners. And a lot of the old 3-4 floor wood rowhouse type buildings were tenements. You still see surviving bits of that higher density wood row housing in some areas.
I wonder what would have happened if the 60's money for utopian projects had just gone into fixing up the old buildings and building new buildings on empty or underused sites, without the social engineering mass demo aspect. I would imagine there isn't much difference between those pictures above and what some now-gentrified areas looked like back then. Another big difference would have been to put in transit instead of trying to retrofit the old road network for cars (there was also a lot of demolition of anything that made the street network irregular).