The trend of suburban areas moving toward Democrats and rural areas moving toward the GOP has been going on for about the last two decades.
No it has not been going on for that long. A large extent of this happened only in 2016.
Los Angeles, Austin, San Antonio, Houston, Dallas, Chicago, Philadelphia, Pittsburgh, Detroit; the suburbs of each of these large cities all trended Republican in 2012.
No, it wasn't just 2016. If you look at trend maps from 2000, 2004, and 2008, in particular, you'll see that a number of suburban counties in those metro areas trended Democratic. The 2016 election was when this trend became more pronounced (and it was countered by a pronounced rural trend toward the GOP). However, the suburban move toward the Democratic party has been gradual over the last two decades.
