Mostly urban density ideas were incompatible with green space in the decades prior. The more you want to fit people to "lower" rent, the less you can allocate for grass or trees.
I don't know. When thinking about "urban density in decades prior" the first thing that comes to my mind are the "commie blocks" of the 50s-80s. The thinking of those often explicitly included green space: put people in denser higher buildings and you have space for green spaces in between. The execution wasn't always stellar, but it was in no way incompatible with trees.
Those projects are common all over the eastern US at least. Western US made them more squat generally a couple stories, but about as much green space surrounding the property. Not often as many trees but the children play outside and leave bikes and toys across the property.
