icecrack

The problem
While spurred by recent events, this essay touches on something that seems to have been a pattern in site moderation for some time now.
Let me make a claim: if a rule, especially one that is vaguely worded, is not enforced, for a decent amount of people that rule does not exist. This nonexistence integrates into the mental model of the rules that forum members construct, no matter what the formal rules may say. For members using this mental model, beginning to enforce a rule that was previously unenforced is equivalent to creating a new rule. As such, the same procedures as those used to notify forum members of new rules should be applied, possibly with some adaptation on the lines of "we will now actually enforce this rule", as the rule effectively did not previously exist.
The ur-example of this is the loligeddon of yore. The takeaway from the loligeddon when it comes to this essay is this: mods repeatedly stated that no rules had actually been changed. Yet nevertheless the appearance and subsequent removal of a particularly problematic post sparked sweeping policy changes, a cleanup operation, a tl;dr post by the administrator explaining the changes, et cetera. This should make it clear that changing policy is a big deal, even if no written rules actually change.
Recent policy changes, however, have been very different from what happened during the loligeddon. Frequently the only indication that effective rules have changed has been moderator action, sometimes fairly strict. In essentially all cases this action has been explained either inadequately or, most commonly, not at all. When this occurs the target(s) of moderation will likely feel that they have been unfairly, erratically targeted by a capricious, uncommunicative bully. What do you do when you get bullied? Well, you could talk to HR, but the mod that bullied you is probably in HR anyway and you might not even know who did it. Another option is to fight back. You annoy me, I pay you back in kind - and if I can get some fun at your expense, sure, why not? You're a bully, you deserve it.[1]
I do not mean to suggest that we need to have a tl;dr writeup every time a rule is changed, but a simple statement of intent would be appreciated. I estimate that writing this should take no more than 20 minutes. As an example, here's a hypothetical notice regarding the changing of rules on gifs that took me ~10 minutes to write. Note that the policy mentioned here could be reversed or altered to be more specific if it turns out that it was unclear or did more harm than good, which is arguably more difficult to do if the rule has been made official.[2]
In the light of this, I would like to present some recommendations.
Recommendations
When moderating, consider if your action is effectively creating or modifying rules
Remember: in the minds of some of your users, unenforced rules may as well not exist. If you decide to moderate something that was previously typically not being moderated, this will cause confusion and consternation.
As such, whenever you make a decision, ask yourself: am I changing the rules? If so, you need to consider both whether your action is actually justified, and how you are going to inform the public of your policy change. You are not a cop, you are a judge in a precedent-setting court. This is especially true due to the (understandable) current policy of supporting other mods' decisions near unconditionally.
Do not make controversial decisions when following up is difficult
On some occasions moderators have moderated while on vacation, using their phone, with bad connections et cetera. I strongly recommend against making anything close to a controversial decision in these conditions. You will end up both ruining your vacation and doing a bad job.
Talk first, shoot later
If you are performing a moderator action which reasonably should include notifying the target of the action, write up the informative PM or otherwise establish communication before enforcement. You could also consider writing up the notification of intent to change / differently enforce / clarify rules before moderating. Most of the time nobody is harmed much by leaving something up until you can handle it properly. For things that require more urgent management such as a fast-evolving derailment, consider either using a PM template for 1-2 people or making a post stating that you have removed derailing posts in the thread you moderated.
Make people feel heard
One key theme of this essay is the importance of communication. This extends beyond just notifying people of changes to the rules. I am under no illusions that your actions will go uncontested or that people won't meme and fling shit at you even if you try your best to communicate as advised in this essay. In part this is due to the frustration some people, and certainly I myself, consider you responsible for creating due to your actions up to this point. However, when hostility meets well-practiced civility its fires often run out of fuel. If you constructively engage with those who would oppose you, you can both soothe their frustration and create better, more precise final rules.
Obviously there has to be a limit and ultimately you set the rules to follow. But explaining, refining, and justifying your position elevates it from that of a dim-witted bully with little justification for their actions to someone who has a well-grounded but different opinion of what the rules should be. The first one deserves punishment, the second, grudging respect.
As a personal observation: in general, you should assume that much less of your decisions are obviously justified than you currently think. One man's common sense is another man's borderline acceptability is another man's utterly idiotic rule enforcement.
Moderation is a hard job
If this all seems like a lot of hard work to you, congratulations! That's what I thought too when the mod applications came along, so I didn't apply. Any moderators that cannot actually moderate disputes should either confine themselves to routine, uncontroversial moderation tasks or step down from their position. Believe me, nobody will die either way, and you'll get to spend your free time doing something that suits you better.

Notes
[1]
I personally don't consider the mods bullies when I do this kind of thing, but I do consider them deserving of public ridicule. The intention is both to correct behaviour and to extract some entertainment out of people that deserve to be made fun of.
And yeah, I have no respect for authority. None. I will judge you by your actions alone.
[2]
This is an assumption based on my conception of normie considerations like pride, sticking with your decision, whatever.
Obviously if a rule does more harm than good it should be removed whether or not it was enforced temporarily, but it is probably easier to do so politically if it was in fact considered temporary.
Look, I'm trying desperately not to kill all normies every day here. Give me a break.