Catching Arceus is now possible by using the Pal Park's retire menu. In the void, if you find a Pal Park activation space, and retire in a Hall of Origin area, this activates the 4th script in your current area, which in this case is Arceus. Then simply go to the Pal Park Activation tile again and retire. If you need more info, check out Crystalmourne's video on YouTube. Hope this helps.