I know some subreddits have "be kind and respectful" built into their rules, but this is often gamed by some redditors who will make carefully worded passive aggressive statements at ideological opponents and then quickly call for banning them when they react. There are also moderation issues with volunteer moderators sometimes using different standards of "kind and gentle" based on whether they agreed with the views of given posters. It seems hard to maintain standards like "kind and gentle" in any objective manner.