Banner

The Banner is a blacklisted, unobtainable item that was previously obtained using exploits.

Properties
This banner was obtained by using the telekinesis enchantment on a banner at spawn in the dark auction when the enchantment was added. More recently it could be obtained by breaking the banners of the traveling zookeeper (which was added in the Pets Update (0.7.6)) which would spawn on loaded public islands.undefined

The reason Banner was blacklisted is it was added in 1.8, crashing any 1.8 players if this item showed up in their inventory, dropped on the ground as an item form, or held in the item frame. Just a few players had the Banner before Pet Update, but then due to a glitch way more players obtained it. This event forced staff to tag Banners as Outdated items, making it impossible to transport them, sell them or keep in an item frame.

Another theory is that, when a Banner is mined with a telekinesis tool, the game code will read what block was mined along with its data and put it in the player's inventory. This is not a problem with the majority of blocks, but when it comes to banners, the game gives the banner in its block form to the player, but that doesn't exist.

There was only one way to keep Banners from disappearing, place them down before the update.

Trivia
The first player to find out about the banners' behaviour is Derpero.

At unknown update, two of the banners that were invisible for 1.8 clients on TimeDeo's island became visible, meaning that developers might be fixing banners so they could be used normally.

TimeDeo's video showed the NBT data of one of the banners at the end of his "how i lost 800,000,000 coins" video. As every other item in Skyblock, the data includes ExtraAttributes:{id:"WALL_BANNER"}, which classifies the item as wall banner (which, in vanilla, does not exist). ExtraAttributes is not used by Minecraft, but the plugins that the server runs on, which instructs the plugins to display that item in the player's inventory.