Honestly I don’t know if it’s something Lemmy-only, but I see many post titles ending with the world rule for no apparent reason
It’s just a rule from those particular communities. You have to post something before you leave and it has to have the word rule in the title. It’s just a silly thing they do.
It’s specifically 196, which comes from the reddit 196. If you see it elsewhere it’s just due to memeing
It’s a community an instance with a rule that if you log in to the website, you have to make a post before you leave (ie log out). The “rule” bit is people indicating that the post is because of the rule, and it caught on. Irritatingly enough.