I thought that Ikoria was the only plane with non-Eldrazi non-artifact colorless creatures, so I looked it up, but it looks like there are actually a couple other exceptions to that rule. Two creatures related to Ugin (makes sense), two creatures really only made to improve commander draft, and [[Wandering Archaic]], which is pretty unexplainable. I feel like it will stay as something “special” for Ikoria - I’d guess we’ll return eventually.