It could be found in the game's files from demosv0.8a until v0.9b, alongside Lake of Rage, Butter Building, Castle Daein, Water Temple and Ice Climbers. Skyworld is much more finished than the other leaked stages, and even has a separate DAT archive of its own in v0.9. However, it may or may not be a future addition in the game.
Three platforms make the infrastructure of the stage along with a lower platform that can be used to aid a recovery. Palutena's Castle is in the background. In the Brawl version, the platforms would break, if they sustained enough damage. It is currently unknown if the platforms will break in Super Smash Flash 2.
Screenshot from Ya Dad's livestream showing Skyworld on the far left, grouped with the other classic stages.
Stage Icon found via data mining.
Stage Selection image datamined from v0.9b.
As seen above, in v0.8a of the demo, a glitch will sometimes cause an inactive thumbnail representing Skyworld to appear on the stage selection screen which hinted at this stage's possible inclusion in a future build of the game.
Skyworld is listed as DAT44 within version 0.8b's game files, as DAT33 in v0.9a, and DAT26 in 0.9b's.
Skyworld contains "Title (Kid Icarus)" from Brawl as its music, but goes unused. It was reused on Palutena's Shrine stage in Beta. In Beta 1.1, the track is now unused, as it was replaced with a remix track.