A perfect rule (or rather three rules) to use on such a platform is the "Three gates of speech" rule composed by an ancient mystic and poet Rumi. It goes like this:
Before you speak, let your words pass through three gates
At the first gate, ask yourself, “Is it true?”
At the second gate ask, “Is it necessary?”
At the third gate ask, “Is it kind?”
Only after the whole three rules are met, you can allow yourself to speak. Words have great power to both create and destroy. It's better to say less and be constructive than say more and cause disbalance and suffering.
How would it work on the platform:
Before posting anything on the platform a notification would pop up "ask yourself if what you wrote/said is true", after pressing "yes" another notification would pop up "ask yourself if it's necessary" if you pressed "yes" again the last notification would appear "ask yourself if it's kind". Only after pressing "yes" three times, your content would be posted on the platform.
The "is it necessary" part is a bit tricky in this context and I'd say, perhaps, generally in life, since very little of speech is actually necessary. And it's highly disputable what is necessary and what is not with regards to human conversations. Most of the conversations are done for entertainment purposes. Is entertainment necessary? You could say both yes and no, depending on the perspective.
So I guess it would be better to leave the middle rule out and only ask people if what they say is true and kind. Even if it's not necessary, since it's true and kind it shouldn't do harm to anybody.
People should be held responsible for their agreement to the rules of truthfulness and kindness, that is if the moderators (or other users of the platform or both, depending on the way the platform was organized) would detect that what they said is after all not true or/and not kind, their post would get removed, if they continued breaking those rules, they would be banned from using the platform.