This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
The leftover is a joke block introduced in Java Edition 20w14∞ that forms the "zones" in the "zones" dimension.
The leftover cannot be obtained as an item even by using the /give command. The leftover can be placed using commands such as /fill or /setblock, and naturally generates in the "zones" dimension. It cannot be destroyed by tools or explosions.
Natural generation[]
Leftovers naturally generate in the "zones" in the "zones" dimension, forming areas of danger throughout the overworld-like environment.
Usage[]
The leftover is non-solid, and invisible apart from emitting "Happy Villager" particles. An entity entering it is inflicted by Blindness II and Poison IV for 3 seconds each. It is not possible for other blocks to be placed inside of a leftover, nor can pistons move or destroy it. When an item is thrown into it, it affects the gravity of the item.