No, withholding pay for failure to participate in an exit interview is totally illegal. They can offer incentives to participate, but they cannot withhold due compensation. Whether you do the interview is optional and up to you, but regardless, you have complete control over what you say during the interview, so you never have to bring up problems. It’s rarely a good idea to bring up what you think are systemic problems during your exit interview; there is usually zero upside for you to vent your feelings, and you could accidentally open yourself to liability and/or to reputation damage with your future employers. The time to speak up about problems is long before you leave, while you’re trying to participate in the solution. The best “stated” reasons to leave are because you found something that better fits your own goals, something that is out of the control of the employer you’re leaving, and not because you’re fed up with their inability to fix something (even if it’s true).
IME even if they’re mandatory you can give BS answers that sound plausible but don’t burn any bridges. Of course, you should weigh this with improving your coworkers experience and the value of any stock you may own (a high turnover rate in a knowledge-based industry is generally bad for the business.)