Then pensioners would move out of state. Illinois couldn't tax those people.
I really need to get away from states like Illinois
Their lib friends in DC tax out of the country earnings all the time. Liberals think they can tax ANYTHING. Alinsky them by enforcing their own beliefs. It's only fair. If they won't bargain in good faith and won't let you revise or otherwise get around the letter of the law, including the Illinois constitution, implement a windfall pension tax. And implement it as withholding, i.e. you take it out automatically when you disburse the pension; they can always file to get a refund - next year - if they have overpaid. And throw their own class warfare rhetoric back at them when they squeal like the stuck porkers they are.