eg. suppose you have a few lines of code that specify that an item only drops if a mob is killed while the player is holding a stick. Currently this only works if the player is holding that stick in their main hand. AFAIK it isn't possible to create a script where the player either can hold a stick in their offhand and get the drop or where the drop will only happen if the stick is held in the player's offhand.

You are right. I can add support for that. I will not modify 'helditem' for this as that could cause compatibility issues with people that don't want that behaviour. I'll add a new tag specifically for this

Next release will feature two new tags: 'offhanditem' and 'bothhandsitem'

ooooo neat thank you.