What exactly is the rule? In the past I've seen claims about what FAA Security Directives required, where those claims later turned out to be false. (e.g., Someone claimed that the SD says the airline must insist that passengers do X, whereas when I later saw the text of the SD, the SD just said that airlines must ask passengers to do X, but never stated that airlines must enforce that.)

Does the SD require that airlines prevent all passengers from using restrooms outside their ticketed cabins? I'm skeptical.

For instance, on a recent flight, flight attendants announced that folks in economy class should not use the cabin in first class unless a meal cart was blocking their way, in which case they should avoid congregating near the first class restroom and if there is a line, passengers should form a line and wait in economy class. That sounds like the kind of thing I can imagine a Security Directive stating -- and it is subtly different from requiring that no passenger may ever use any restroom outside his/her ticketed cabin.

(Let's count the differences: (a) the difference between not allowing economy passengers use of first-class restroom, vs not allowing first-class passengers use of economy restroom; (b) the difference between not allowing economy passengers to use the first-class restroom, vs allowing them to do so as long as it does not create congregation near the first-class restroom; (c) the difference between one's ticketed cabin and the cabin where one's seat assignment is (e.g., upgrades). There are probably more.)

Details matter.