My understanding is that it means going, loosely, to the opposite side of the state of the major metropolitan area in that state. Upstate NY is the northwest part, upstate MA is the west part, upstate PA is the northeast part. I’m looking around, and it seems to also 1) only be used on a few states, 2) usually is on the north half (but not always), and 3) is somewhat interchangable with “rural”.
regardless of what you think of him, Deng Xiaoping was the guy in a way that pretty much no other human is gonna be for the unfolding of the 21st century