Well, I finally have some definitive news to share concerning KB4507704; unfortunately, it's not the news I was hoping for.
After much digging, I have come to learn from a credible source that Microsoft did indeed produce KB4507704 for Windows XP Embedded POSReady 2009. The notification stating it is available for POSReady 2009 through the methods listed in it's support article is, obviously, inaccurate because Microsoft never followed through with their official formal process of releasing KB4507704 for POSReady 2009 through Windows Update, WSUS or the update catalog. As it stands now, because POSReady 2009 has reached EOS, the only way to gain access to it is to open a paid support ticket with Microsoft in order to get it released, despite them being the ones who published the erroneous information in the first place. Unbelievable.
So there you have it. All the speculation and theories put forth here have proven to be correct. Thank you and good work everyone!
Not to worry though, if you're a Microsoft Premier Support subscriber/member/partner, you undoubtedly already have KB4507704 (along with all the other security and non-security updates released over the past year) installed on your Windows XP/POSReady 2009 systems. For us franchisees of the Casablancan Supermarket chain, thankfully we have been graciously provided with a custom registry fix by @win32 to help us not only accomplish the same thing as KB4507704 but with the added benefit that it can also be applied to all NT 5.x OSes.
Finally, I would just like to close by saying that Microsoft should be ashamed of themselves for continuing to have information publicly available on their own website(s) that give the impression this update is easily obtained from methods we've been accustomed to utilizing over the years. If they aren't going to ultimately release KB4507704 for POSReady 2009 to the update catalog, then at the very least they should correct the information on their support page(s) to reflect this.
Tsk. Tsk.