I did not know this. Previously, I had predicted that Chicago won't go the way of Detroit for a long time because there are still plenty of wealthy people to fleece. As long as they can still cut checks, they can still cook the books. I had surmised that until you start seeing a trend of population decline, Chicago can't even be considered to be on the list of who's next.
This must be a well kept secret. Because it is the upper and middle class that can afford to move or relocate. The poor don't move. The wealthiest usually go first as many of these folks already work on a national level. If they are losing population, it's a good bet that many of these are the highest tax victims.
I know a lot of Chicago people move to the high rent districts in northern Indiana (just out of Chicago). But those folks get hit with double taxation from IL and IN. An IL law requires workers that earn income in IL and live in IN still have to pay IL income tax. IN also collects taxes from them but I think IN let's them take some part of a deduction for what IL charges them (Not sure of the details here). I speculate that is why there is so much money in the Region (far northwest IN).
What happened with Detroit is that the big businesses stayed downtown but their employees moved outside the city limits and commute. All those little neighborhood businesses followed their customer base to just outside the city.