Unused content (SSBU)
Considered for the base game
Considered as DLC
Compared to previous games, Ultimate has significantly fewer unused voice clips. Characters whose voice clips are ported over from Super Smash Bros. Brawl have almost all of their unused voice clips (such as Captain Falcon's soft "Yes!" and various short clips spoken by Peach and Luigi) removed altogether from their sound files (with the exception of a few voice clips of Snake, which are repurposed in the game); this is slightly odd, as these characters did not have any voice clips exclusive to their character customizations in Smash 4 (e.g. Palutena's Heavenly Light over Autoreticle) whatsoever; and despite Kirby, Pit and Lucario (three of the characters who had unused voice clips in Brawl) had their voice clips re-recoded in Smash 4, the other characters' voice clips were left almost untouched (aside from one of Pikachu's voice clip and Link and Captain Falcon's heavy knockback voice clips).
For a list of unused Spirit Battles in the game, see Unused content (SSBU)/Spirit Battles.
Although every spirit in the game is obtainable, there are many that can never be battled, as they do not appear on the Spirit Board and cannot be battled in any other context (such as World of Light). This is true of every spirit that is an enhanced form, as well as many that can only be obtained by summoning. Most of these spirits have default placeholder battle data, but some of them are modified from the placeholder data to varying degrees. This indicates that it may have been intended for these battles to be accessible before they were cut from the game.
In addition to spirits whose battles cannot be accessed, there is data for "dummy" battles as well. These battles were likely used for testing during development and were not intended to become complete, accessible battles in the game.
Artwork & Alternate costumes