Keep Inventory must be enabled to start with or it will not work. To do this run the command /gamerule keepInventory true

Inventory Drop Toggle
This is a basic plugin that allows minecraft uses to set their own keep inventory rules.

Commands
/inventorytoggle - toggles keep inventory on or off for the player running it
/it - toggles keep inventory on or off for the player running it

How so set another players
Go into the Inventorydroptoggle directory and then the Playe Data. Find the uuid of the player using a tool like MCC UUID to find a players UUID.Then edit the drop value to True - keep inventory off, or False - keep inventory on. This will only work if the server is offline and you must restart or reload after.