The only way to get rid of the feedback messages, a question I often get all over social media, is a clumsy workaround (will post a blog post and add the link here). To achieve this for a complete quiz, you can take out the Default labels, and set the system colors for Success and 'Error' (why do you use that term is a mystery) in the Theme to 0%. But... contrary to previous versions where taking out the feedback messages resulted in getting rid of the second step of the Submit process (click on slide or Y) tthat is impossible in this new version, which means that learner has to click a second time on the quiz slide. That was a very complaint in all versions, complaint is popping up again.
Getting rid of the feedback messages on individual slides, which could be done very simple in CP2019 by unchecking Captions and setting Failure captions to None needs to set the color of the Text message to the background color and and the Alpha of the fill color of the states to 0%. Same result: learner still has to click a second time to proceed. I tried multiple workflows to avoid it (like changing the label on the Submit button) which are just impossible to achieve.