The power-up used to be an Anvil, but it was replaced with a Hammer in the final version.
A telepod was supposed to be available in December 2014 to unlock an additional costume for Poppy (Spring Day costume), but it was never released. While the costume art is finished, the related photograph is unfinished. Two other costumes for Luca are unused, such as a black beanie and a yellow afro wig. These can be found in the game files.
Unused Spring Day Poppy Photo
Spring Day Poppy and telepod base
Spring Day costume for Poppy
Yellow Afro Wig costume for Luca
Black Beanie costume for Luca
There are also some telepods that have prototypes but do not have sprites in-game. These were seen in a display case in Toyfair 2014.
Willow with yellow glasses
Gale with a pink crown
Poppy with pink shades
Dahlia with a white flower on her head
Chapter 3 (Caves) and Circus level
After the release of Angry Birds Stella, a third, unnamed cave episode was teased at the end of the game's second episode that says "Coming Soon", same goes to Circus level in v1.1.1 update. But unfortunately, both of them were scrapped as the game was discontinued.
There are some Lost and Found items are unused and unfinished (such as the Hanglider that later appeared in Own The Sky, Stella's headset, a Pig Mask from the Ham'o'ween Short Movie, Luca's Toys, a Record Player, Dahlia's Science Kit, Posters, Whoopy Cushion, etc.).
Unused tutorial sprites of Stella and her friends.
Unfinished cutscene for Chapter 2 with unseen panel on the top right corner.
Unused app icon
Unused Stella Photo
Unused Lost & Found Items
Some of Lost & Found item icons are unused.
Concept art of Lost & Found items.
Possibly tutorial sprites
Unfinished Chapter 2 cutscene
Luca's unused costume, ticket, and small gold pig icon.