Do you mean skin for 21:9 screen? I dont think there's any, because mostly players play with 4:3 or 16:9 screen. Maybe tweak some skin so it fits with your screen or play osu in letterbox 16:9 resolution.
There is a handfull of others, but I don't remember them. Also pretty much every skin that doesn't do any fun gimmicky stuff should work, thought this is mostly older skins