Stacked Items
Stacked Items are items that are illegally stacked, generally consisting of tools, armor and weapons. Back in 2021, these could be obtained using the Crafting Table Dupe.

History
Early or Mid 2021
Crafting Table Dupe was added, which allowed illegal stacked items to be created.
This also included totems, and for a period of time, PvPs were practically infinite. However, this was eventually patched.
Mid 2021
Crafting Table Dupe got heavily nerfed, then a few days later the change got reverted (exact date unknown).
It should be clarified that also, on an unknown date, stacked tools, weapons and armor got patched. Almost everything except shulkers.
Present
As of 2026, stacked items no longer exist and are not possible due to plugins like AEF.