Adding notes to products for internal use is unavailable. I'll surely send this feedback to our product developers. We gather data about features the customers needs the most and do our best to include it on the next updates.
This feature used to be available, and I now notice it has been removed. I used it to add the weights of products we sell to use to calculate postage. I see that all my weights have been removed and I am going to spend the afternoon weighing things.