ppl would eat crab apples from trees. In retrospect, a lot of urban soil is excessively high in lead. But it’s maybe possible to just plant some apple trees and they’ll eventually provide some food security.
Also would make those empty lots into more park like structures
Most of the abandoned plots are sadly not suitable for this, but there have been certainly attempts to do things like this and there is a whole bunch of ideas related to “seed-bombs” that go into a similar direction.