BETA PREMIUM New activator EI_LEAVE_THE_PLAYER_INVENTORY, EI_ENTER_IN_THE_PLAYER_INVENTORY
These activators has been created only for the non-skackable item.
It will allow you to run a command when the EI leaves the player inventory, if the EI is moved in the inventory, or if the EI enters in the inventory.
(it supports detailedSlots option)

By combining both you will be able to make items that gives boost (effects, attributes,..) without using a Loop. And generally will allow you to do more things

It's in beta because there are a ton of leave and enter cases so I probably forget one

Item to test the activators : < https://pastebin.com/QYpuFCJk>
Example item (Nether Star that gives strength boost when it is placed in the offhand) : < https://pastebin.com/GfHKdZak>

CHANGEMENT for the commands recently added:

NEAREST {commands} -> NEAREST {max accepted distance} {commands}
MOB_NEAREST {commands} -> MOB_NEAREST {max accepted distance} {commands}