Interestingly enough, back in the early nineties, when I was working in a print bureau, the vendors would warn us how traceably colour copiers/printers of the era were, so it seems like an example of an "open secret".
Same. When the company I worked at in the mid-nineties got our first color copier, the sales rep told us explicitly about the yellow dots, including how to use the zoom and contrast features to see them (i.e copy a copy on the highest level of zoom). They just didn't want us copying banknotes, which was, of course, the first thing we did.