Key Items
There will be a special Item Inventory, usually with Items you start with or that custom Scenarios can give you.
When you respawn you will still have said Key Items, and you cannot drop Key Items on the Ground, in the World, nor into Chests.
The only special thing about Key Items is that they are in the Key Item Inventory. Any Item could be placed into said Inventory (using Startup Items, Commands and Cheats).
A typical Startup Item could be a Music Player, which would be the way you enable special background Music ingame (but only for yourself) and have no real effect on anything. (there will be a way to compose Music ingame similar to Noteblocks but better)
Same can go for certain types of decorative Clothes, which some people ofcourse dont want to lose on respawn.
It would for example be possible to have a Spear in the Key Items Inventory, and you could melee attack with it, but you could not throw it at people because Key Item, unless it would be an infinite Spear Item.