Is UFO corki one of the ultra rare skins returining?

We dunno yet. They haven't said anything about him returning.

-Limited Edition skins that were once in the store but are not part of these events (Rusty Blitzcrank, Urf Warwick, Championship Riven, Riot Singed).If these skins are ever brought back, they will be treated similarly.

-Earned Limited Edition skins that were rewarded for events or achievements and never appeared in the store (PAX Twisted Fate, King Rammus, Victorious Jarvan, UFO Corki, etc.). We have no plans to make these skins available.

http://na.leagueoflegends.com/en/news/store/sales/plan-vintage-limited-edition-skins

This was 4 years ago, and could change. But it seems like they're still going on this path. Other than pax sivir. But that skin got a chroma.

Ufo corki will probably never be available, but maybe a chorma or something like that.