Strongly disagree with this point. Your departure should be an enormous shock to management, otherwise you've just spent weeks or months mouthing off about work to them, they've not fixed anything and indeed might even have lined up your replacement already.
> Instead, you should make sure that if you’re unhappy about something, your boss knows it pretty much as soon as you do.
Keeping a paper trail of those discussions also help even if it's just for sanity check for your future self.
Chances are most people won't help you because most people are bystanders or cowards in those situations (anecdotal, maybe everyone I know and I are unlucky).
If you have concerns and constructive feedback, I found most managers to have agreeable reactions to it.
Maybe I was just lucky, but I did have this experience across four companies. What you should not do is bare criticism without constructive feedback. Most managers will respond positively, even if nothing happens in the end. And after voicing it for several times, you can feel better for leaving, because you also did what you could from your end.
“Everything is fine” until a key milestone is hit - it’s time to move on, or there’s an internal role you’d like to apply for.