Since the beginning of Minecraft Dungeons' development, there have been a number of features added to the game that have no actual use in-game. Some of these features are leftover from removed features, or simply placeholders for new features.
Mobs
Jack O' Lantern
The Jack O' Lantern, otherwise known as the Jack O' Lantern Trap, is an unused mob that was added in the Beta stage of Minecraft Dungeons and was planned for the Jungle Awakens DLC. They were scrapped at some unknown time but they can still be spawned in by using commands
Mason
The Mason is an unimplemented NPC mob that would have been able to upgrade the players gear to their current power level using Diamond Dust that could be bought for 1000 emeralds. The developers weren't too fond of it, so they scrapped it mid development. Its item-upgrading functionality was re added in the Creeping Winter update with the Blacksmith, minus the diamond dust.
Mage
The Mage is an unimplemented spellcaster Illager that was planned for the Creeping Winter DLC, but it was scrapped. It has been in the files since the game's release (1.1.1.0), but was removed for no apparent reason in the 1.2.0.0 patch. Based on its appearance and its spell casting abilities, it was likely going to be in the Iceologer's place.
Quest Giver
The Quest Giver is an unimplemented NPC mob that would have been able to give quests to the player similar to the mission map in the camp. It was first added during the Creeping Winter patch along with the other new camp merchants, with it potentially being one of them, but it was never implemented. Its appearance resembles the savanna villagers from regular Minecraft with it having sandals and red and green clothes. It also has a bandanna over its eyes and wears a crown.
Items
Curious Armor
Curious Armor is an unused unique variant of Guard's Armor that had the unique ability to have a 5% chance to teleport players when they took damage. It is unknown whether the teleportation ability is what made it unused, but the teleport function cannot be obtained even if the player manages to get Curious Armor using hacks.
Diamond Dust
Diamond Dust is an item that could be bought and used by the mason to upgrade the power level by one of a piece of gear (to a maximum of level 112).
The Monkey Motivator
The Monkey Motivator is an unused unique variant of the pickaxe that was planned for the Jungle Awakens DLC that would have features a banana on the end of a stick and would possibly make a monkey sound when used. It can be obtained with hacks but will cause the game to crash whenever its clicked on.
Totem of Soul Protection
The Totem of Soul Protection, known internally as the Totem of Spirit, is an unused artifact that would have the ability to fully revive all players in a square area upon death, similar to the base game's Totem of Undying. It was actually showcased in the Minecraft Dungeons Diaries episode Meet The Team, where it would produce a aqua colored square area, but hasn't been mentioned since. The artifact's death-cheating function returned to the game in the form of new enchantments called Life Boost and Death Barter, released in the Howling Peaks DLC.
Enchantments
Altruistic
Altruistic is an unused powerful enchantment that would heal nearby allies by how much damage the player takes. This enchantment is similar to the "Heals Allies" enchantment given to mobs.
Barrier
Barrier is an unused enchantment that was planned for the Jungle Awakens DLC, but was scrapped along with the Jack O' Lantern and The Monkey Motivator. It would increase the players damage resistance per number of enemies nearby.
Regeneration
Regeneration is an enchantment that would allow the player to passively regenerate health after 10 seconds of not taking any damage. This enchantment can still be applied to mobs however.
Shielding
Shielding is an enchantment that would give an aura that gives damage resistance to the player and any nearby allies. In 1.7.2.0, mobs were given the ability to spawn or be enchanted with Shielding.