I am confident this is a rule enforced by the moderators but does not appear in the OP.
"34. Discussions about the moderation of threads, or the forum are automatically off-topic in all threads except threads located in meta"
I think it would be beneficial to add the above to the OP. I don't think everyone is aware of this rule, and adding it should cut down on these rule violations.
While discussing moderation in a thread not about moderation is
usually off-topic, it isn't automatically off-topic. Whether it's off-topic depends on the context - someone complaining that "X thing is moderated and shouldn't be handled by the community" isn't necessarily off-topic in a Reputation thread about thing X but someone posting a complaint about his / her posts being deleted in a non-Meta thread they were deleted from is usually off-topic. Context is also massively important when it comes to off-topic content as well - a throwaway line about moderation in a massive post that is completely on topic doesn't always warrant deletion or changes. Obviously, that's my interpretation of what is and isn't off-topic in such cases which brings me to another issue with the proposed rule - what is considered off-topic is up to a moderator's discretion. Considering how subjective this rule is, trying to codify it too much will lead to an inaccurate view of the rules. To quote an older post of mine about a similar issue:
As with automated translations, in the end the rules are enforced by moderators. Me trying to zero in on what is and isn't plagiarism more than I already have would only reflect my own approach towards the enforcement of said rule. As theymos mentioned numerous times, there are reasons as to why there are no "official, hard rules (aside from the few legally-required ones)" - it's up to individual moderators to decide whether acting on amperceived violation of forum policy is the "right" thing to do. As such, I don't think there's a need to expand or adjust rule 33, at least not at this point in time.
tl;dr The proposed rule doesn't account for the mountains of context / subjectivity of off-topicness and off-topic posts are already covered under rule 2.