I would argue that the unsanctioned permanent slums are already present in Seattle, Portland, Los Angeles, San Francisco, and other cities on the west coast.
They're not yet at the scale of slums, favellas, barrios, etc., of third-world cities. For the most part that role remains the domain of "retirement" communities of the less-than-privileged sort, usually in outlying regions or depressed (e.g., Rustbelt, rural Sunbelt) areas.