I think that's the 196 community. One of the rules is that if you show up, you have to post something, so my understanding is that people just put "rule" in the title to say that they're complying with that rule.
one specific community, primarily one one instance. The concept of "rule" originates on r/196, where the only rule is that you have to post when you visit.
Therefore, its easiest to write "posting because rule" in the title rather than something creative. AFAIK this eventually devolved into just saying "Rule" or some sort of play on it.