Ah yes, photo paper with a “silk” textured surface. Anyone here remember it, or use it in the past?
Yeah, it was fairly common for portrait work circa 1970s, maybe into the 1980s, at least in the US. I don't really know the history of how this came about; I just presumed that the sense of being a more "luxurious" sort of surface helped to justify charging a higher price. And, like other "professional" textured papers, prints could be viewed from nearly any angle without excessive "glare" being reflected. (In the case of glossy paper, for example, you don't wanna hang a framed print on a wall where the viewers have large windows behind them; the shiny reflection could make it nearly impossible to see detail in the darker parts of the print.)
FWIW the silk surface really killed off fine detail, so it was not very suitable for small prints. (I'm speaking of the surface called 'Y' by Kodak.) Part of the effect was also to swallow up film grain, so one could sorta get away with using smaller film.
You mentioned the difficulty for someone trying to make (unauthorized) copies. The 'Y' paper surface had an array of tiny dimples sticking up. A print could be copied easy enough, but the result showed the dimples. So if someone printed a copy neg onto a gloss paper, for example, the uneven dimple effect showed up, making it fairly obvious that it was a "pirate" print, not one from the original photographer.
Fwiw, circa 1980s as I recall, a photographic trade group (the PMA, I think) did a "sting operation" on some large outfits that specialized in making copies. After some sizeable lawsuits for making copies of portraits that were marked with a copyright symbol the copy specialists started to police their own industry. Such that if someone brought in an apparently professional image to be copied, the copy shop would typically want to see a "release" from the copyright holder (the professional photographer). Around this time, I think, the silk texture papers sorta went out of style, which I think may be related. Or maybe not. I would say that the Kodak 'E' surface finish sorta become the de facto standard for such work.
I'm from an outfit that owned a large portrait studio chain, so had quite a lot of experience with this sort of thing.