I would recommend you don't apply this registry "hack". While Windows XP POSready edition did have a longer support cycle compared to Windows XP, there are some updates that were released specifically for POSready and aren't designed for machines running regular Windows XP. Installing those updates can cause things to break.
Can you name one POSready update that broke Windows XP? (Other than KB3013455, which was just a mistake, and was later fixed.)
Agreed, cite sources.
I agree. This is the standard FUD spouted by Microsoft to put people off doing things that would enable them to put off upgrading.
In reality, all versions of Windows XP (for the same CPU architecture of course) use exactly the same kernel. The only difference will be the user interface. And if something does go wrong when applying an update (this happened, even with those updates designed for desktop XP) then it's normally fairly easy to sort out.
In any case, I now use Linux as my main operating system. I only use XP on my old laptop.