This is majorly based on experience and some part is hypothetical.
We generally see that some users rage quit after a bad experience on the site like a suspension or closing of questions. While some other users leave a site or become inactive to balance real life and SE life.
A few users declare it on chat before deleting their account or becoming inactive. I have seen such messages in chat rooms and comments. I have flagged such comments because they add nothing to the post. But what to do when they declare it in a chat room before rage quitting?
I am more thinking of rage quitting because such conversations are sensitive to handle and attract flags. The users are good contributors and having a good name on the chat. But when they start targeting or say objectional messages, it would affect on their reputation (not the points). They accuse some users and cross the "Code of Conduct" before they leave. For example. Is saying something like "Bye. Don't devastate your name before leaving. Have a good farewell then" crossing CoC ? (Honestly without any sarcasm.) Does that guarantee a chat ban?
Should we always try bring the user back and ask to stay when they say they are leaving? Given that we know they have reasons for leaving? (I say this because I remember reading lines like "users are not important. Content is" and saw moderators ignoring messages about users leaving and saying "We can do nothing. We can't everyone". Having said that, I don't undermine their value either)? Is it considered rude to bid a farewell? What is the ideal reply in such cases?