6 files declare their use of CheckoutPanePluginBase in Ubercart 8.4

AddressPaneBase.php in uc_cart/src/Plugin/Ubercart/CheckoutPane/AddressPaneBase.php

CartPane.php in uc_cart/src/Plugin/Ubercart/CheckoutPane/CartPane.php

CustomerInfoPane.php in uc_cart/src/Plugin/Ubercart/CheckoutPane/CustomerInfoPane.php

OrderCommentsPane.php in uc_cart/src/Plugin/Ubercart/CheckoutPane/OrderCommentsPane.php

PaymentMethodPane.php in payment/uc_payment/src/Plugin/Ubercart/CheckoutPane/PaymentMethodPane.php

QuotePane.php in shipping/uc_quote/src/Plugin/Ubercart/CheckoutPane/QuotePane.php