7 files declare their use of OrderProductInterface in Ubercart 8.4

IncrementStockAction.php in uc_stock/src/Plugin/RulesAction/IncrementStockAction.php

OrderProduct.php in uc_order/src/Entity/OrderProduct.php

uc_attribute.module in uc_attribute/uc_attribute.module

Ubercart Attribute module.

uc_googleanalytics.api.php in uc_googleanalytics/uc_googleanalytics.api.php

Hooks provided by the Google Analytics for Ubercart module.

uc_order.api.php in uc_order/uc_order.api.php

Hooks provided by the Order module.

uc_product_kit.module in uc_product_kit/uc_product_kit.module

The product kit module for Ubercart.

uc_stock.module in uc_stock/uc_stock.module

Allow ubercart products to have stock levels associated with their SKU.