Java Edition:Extra Slots Shulker Box
Mining a Shulker Box containing overstacked items from 18w43a (1.14 snapshot) to 24w07a (1.20.5 snapshot) in survival mode will drop a shulker box item with an Items tag with the overstacked items split into separate slots with the max stack size of that item.
Obtaining
To obtain this, you must simply mine a shulker box with enough items to create more than 27 slots (ie: 28+ Diamond Pickaxes, 433+ Ender pearls).
Uses
Placing the shulker box on the ground will remove the extra slot data - however, dropping the shulker box as an item and breaking it (ie. throwing it into a cactus) will drop all of the contents even the extra slots. If there are more than 256 slots, any slots after that will loop back to slot ID 0 and override previous slots when placed.
Observability
This feature is observable by destruction after 20w51a, as throwing the shulker box at a cactus will drop all of the items in the hidden slots. This then becomes fully observable after 24w09a[1].