KUALA LUMPUR—As with past editions of the global conference on cities, the ninth World Urban Forum kept coming back to the stubborn question of what to do about slums.

An estimated 900 million people live in informal settlements, according to UN-Habitat, the organizer of the event, which concluded this week in the Malaysian capital. According to various estimates, one in four city dwellers is in an area lacking basic services, and the majority of all new housing worldwide is technically built ill…