dont know about the horcruxes, but shield would probably know the wizards' existence because most of the higher ups in respective governments know that, as evident in the hp books (Dumbledore and Fudge going to meet the uk pm after voldi's 2nd attacks)

respective muggle* governments

i think shield researched about ancient wizards and having an horcrux was pretty common in many stories in Asian stories especially in Indian folklore. as many kings made them. and had diffrent types as well as some needed special or specific weapons, or destroy them at the same time or diffrent conditions should be made to destroy the horcrux and should they not be met, it doesn't get destroyed or simply gets fixed. I mean voldemort made the most basic horcrux when i First saw the movie. I got this thought.