Via Amazon Seller Central News:
To simplify the apparel product listing experience, Amazon is combining two existing attributes—material_composition and fabric_type—into the fabric_type attribute.
Starting August 20, 2021, you must select the fabric_type attribute when you create a new listing, as material_composition will no longer be available. There is no action required at this time but we encourage you to start using the fabric_type attribute for new listings.
For existing listings, Amazon will copy the current values from material_composition to fabric_type. We’ve already begun this work which you can review in your inventory.