In the Announcment forum, the GM status notice thread at the top says it was last commented on today, but the only message was both posted and modified last in 2008.

I couldn't find anything that was recent, so why does it claim to have a comment from today?

it was just bumped by BBR, due to a conversation in the community chat thread.

Oh, thanks.

Maybe they should say "ladt bumped on [date/time]"

Just a thought.

It was actually modified today though, when I switched it to a Notice, so it would stay on the first page.  So the text wasn't modified, but the status of the thread was.
This message was last edited by the user at 20:57, Thu 20 Apr 2017.

I'm really surprised its contents are news to people.

Well, not everyone reads everything, and them not reading everything is one reason why they might be surprised that they're expected to do so in their own games. But there's also plenty of other things...

Some people only read what's immediately available (this particular thread was page 2 originally, where the internet knows not to go) and that they believe is relevant to them.
It's bound to be new to someone everyday.
Even with giant flashing signs on their TV screens that say things like "We are aware of the issue, please don't call. Your programming will resume shortly." I would get calls from people when I worked Tech Support. Why did they say they called when I asked, you might ask? "Because I thought you should know!"
Too busy planning for raptor attacks.

Two is definitely my favorite of reasons, but take your pick on yours.

Why, yes, I do read xkcd sometimes. Why do you ask?

I'm not even talking about an ability to read everything, I'm talking about the very narrow topic of some GMs not knowing explicitly stated expectations from the site, and how that is dumbfounding.

I was the one that didn't know the specific wording of the rule and thought it somewhat milder (which, based only on reading the items in the site Help and most of the FAQ section is a reasonable understanding).  You say "explicit", but you probably don't realize that said rule was/is only stated in two somewhat obscure places.  One was a 9 year old post on the second page of the announcements forum, the other is deep in the FAQ about mature games and adult content in PMs.  Bumping said post to the top of the announcements forum can only be a good thing.

Now, the rule does come up a good deal with regards to adult games... but there's a lot of potential for folks running only general rated games to not feel that they apply to them.  If you don't run adult games, odds are that the thought has never even occurred to you that you have to watch out for Adult Content Policy violations in your player-to-player PM threads.  Having rules which are potentially a big deal be hard to actually find for new users isn't a good thing.  I'm of the firm opinion that rules should be clear, concise, and readily available to anyone that bothers to look (and, really, the current state with them spread between four or five distinct areas isn't great for the last one).  Anything else just leads to problems.

As the quote goes, "A vague disclaimer is nobody's friend."

Another part of it is people thinking of themselves as being reasonable with good common sense who might simply expect that posting with common sense would be sufficient to avoid infractions.

But sadly, even good common sense isn't enough. I can't tell you how many times things had sneaked up on me in my early days that I totally didn't even consider as even potentially an issue.

There is also the fact that as the number of rules goes up, the longer and more difficult it becomes to remember them all, so it seems quite reasonable to not bother with sections that don't seem to pertain to you. For example, why learn the adult games policies if you never play adult games? And then it bites you in the posterior. :)

Unless you also count all the times it has been mentioned across the forums over the years since the site was set up.

For one thing, it isn't the Adult game policy.  The rule in question applies to every game on the site, regardless of its rating.

Also note that the Adult Game Policy itself applies, in part, to every game.  That part being the section which describes what content requires that a game have an Adult rating before it can be included in that game.

The rule in question is not a new one (in fact, it has been quite some time since a new rule was added to the site - in recent years the only changes to the site's rules have been clarifications, not additions - so the number isn't actually going up).