Item.onCreated gets called only when the item is crafted. Is there a similar method that gets called when the item instance is created, so that it gets called when a mob drops it or you grab the item on creative, etc? If not, how would one go about achieving the same goal through other methods?

Better would be to explain what exactly you're trying to accomplish, Why do you need a per-itemstack callback?

10 minutes ago, LexManos said:

Better would be to explain what exactly you're trying to accomplish, Why do you need a per-itemstack callback?

I just want to add NBT data to the item stacks and it would help keep the abstraction I have going on in my code to have a method where I can place an itemstack instantiation method.

 

It would also work to override a method that creates the default NBT data of the item if there is such a thing.

Minecraft doesn't have 'default nbt', best you're gunna get is the capability init function.