>>33769742You can actually get to Newmoon Island and Flower Paradise without any third-party device in Pokemon Diamond and Pearl.
Since they didn't want the ones obtained this way to be technically legitimate, they instead distributed Darkrai and Shaymin directly for D/P, rather than distributing the items to go battle them. In Pokemon Platinum, the glitch was fixed, and so the event items were given out for Platinum only.
It's been a while since I read about this next part, so I might be incorrect on some of it.
Arceus was never found, possibly because his area or his overworld trigger (The Arceus sprite you walk up to) isn't actually loaded until the Azure Flute is played. It's not known why they never distributed the Azure Flute, even in Platinum.
If anyone wants to know more about this, including how to do it, it's called either "tweaking" or the void glitch.