It's called an exit interview. Once you secure a new job, don't be shy with your managers about why you are leaving.
I would be wary about being completely open and honest about such things at exit interviews. There is nothing to gain for the ex-employee personally at that point in time, and everything to lose by 'burning bridges'. You never know when a few criticisms (even if they are accurate and correct) will come back to haunt you. At the exit interview, the main thing is to do what is best for yourself. If you really want to c…
The point was more, "if you're unhappy, find something new." You don't owe anyone anything, and if they can't figure out how to incentivize you into staying, you shouldn't. It sounds like old manager had a good grip on it and new manager does not, in this particular case.