I managed to track down where this was being called. It was attempting to display the order item meta like so:
So the notice is telling me that this is deprecated now. However, it didn’t really tell me what to do instead.
To save you a bit of time, I thought I’d post the solution here. I assumed that the order item meta would also be displayed in order emails. This led me to track down a function added in WooCommerce 3.0.0.
Instead of constructing the class as above, you can simply do this:
Much easier! You don’t even need to prepare a $product beforehand.