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.
Perhaps the rule could be narrowed such that it would always apply. One suggestion might be to narrow the rule to say that you cannot complain about posts being deleted from that specific thread unless the thread is in meta. Another option might be that it is not acceptable to repost a post that was previously deleted by the moderators, without permission from a moderator, or that you cannot take action to reverse what a moderator did, unless you are complaining in meta, or have received permission to do so.
I don’t see it every day, but there are spirts of instances where someone will complain about the moderation about a specific thread in that thread, specifically they are complaining that posts are being deleted (never about posts not being deleted).
Regardless a one line about the moderation in an otherwise on-topic post, I have seen off-topic divergences that have one line related to the OP, even though the majority of the post is off-topic. I bite my tongue and don’t report these posts because they are technically on-topic. Perhaps some similar standard could be written.
My biggest concern is that someone who is complaining should know where to complain about the moderation. Almost all the time, if a post is moderated, the moderation follows forum policy. However there is always the risk a mod goes rogue, or a mod account gets hacked. If this happens and people complain in the wrong section, the administration will have difficulty detecting the moderator doing something he shouldn’t be.