Technically, a 'slum' is an unplanned area within a municipality. As such, it may not have roads, electricity, water supply, fire/police departments, public schools/hospitals, and etc... The unplanned (and untaxed) nature distinguishes a 'slum' from a very poor city district. China has a zillion very poor/run-down residential areas but no slums due to its strict residential registration ('hukou') system.
Slums are notoriously difficult to eradicate once they are firmly entrenched. Slums don't start to disappear until a country has reached 'high-income' (typically measured by GNI per-capita) level. Given that less than a handful non-city-state countries have managed to escape from the 'middle-income trap' since W.W.2 (e.g. South Korea, Taiwan, and Poland), slums will remain a fact of life in poorer countries for decades to come. You just have to ask yourself what is the prognosis for these low and middle-income countries to break out of the pack. Some countries have been trapped for decades already (think Argentina, Brazil, and Mexico).
China has been an 'upper middle-income' country since 2015 and is on the verge of becoming a 'high-income' country (end of 2021?). This is what China 2025 was all about. Where to seek comparative advantage and increase the productivity level so China can reach OECD level of development. China need these productivity gains to compensate for its aging (and soon, smaller) population.