In northeastern China is a "Rust Belt" of provinces (Liaoning, Jilin, Heilongjiang) with old heavy industries based on iron and coal. The Japanese invested heavily here and the Communists continued to do in the 1950s and 1960s. However, since the beginning of economic reform this region has stagnated. Since 2014 the troubles have worsened and analysts suspected that the population was declining steeply; the news was full of stories about empty schools and abandoned neighborhoods.
The latest Chinese census reveals that population in these provinces is down by 11 million people, or roughly 10 percent. Most of that decline has probably taken place since 2014, so that's a very rapid contraction.
Asked about this, the head of China's National Statistics Bureau said,
The population decline in the Northeast is influenced by a variety of factors including the natural environment, geography, population fertility levels, and economic and social development. The Northeast is located at a high latitude with relatively long and cold winters, and some Northeastern people are migrating to the warmer south. This is a trend of population movements in many countries around the world: both Europe and the US have seen this kind of phenomenon. In addition, the natural population growth rate in the Northeast has been lower than the national average for a long time, because of fertility-related values and behavior. It is also important to see that the economy of the Northeast is in a period of structural adjustment. The economically developed coastal provinces and cities offer diverse opportunities and employment prospects, which are very attractive to people in other regions including the Northeast.
What strikes me about this is how universal some phenomena are in our civilization. National policy may influence these trends by a few percent one way or another, but the main drivers of economic change are largely outside political control: the collapse of population growth as rural people move to cities, the rise and then fall of heavy industry, etc.