Back before Tapas had the check mark boxes for specifically why the chapter is mature, it was common to type the warning in the beginning.
But now it changed and we have the boxes. If you say a few profanity or lightly touch a mental health topic, it could probably not be noticed and you don’t need a warning at all. But if you think it needs a warning, I’d mark it as mature regardless. Even with the written warning, a reader could report the chapter because the chapter itself wasn’t labeled with the red box or the warning screen