You no longer need to set product category manually on the shipment - it is populated from the order and its line items when an order is created, updated, or linked to a shipment.
Product category is now a supported field on order line items. You can assign a product category directly to each line item within a v3 order.
Product category is synced automatically from orders to their associated shipments in the following scenarios:
A v3 order with a product category is created and converted into a shipment
A v3 order is added to an existing shipment
A v3 order with a product category is updated while linked to a shipment
A v3 order is removed from a shipment