7 files declare their use of PaymentMethodPluginBase in Ubercart 8.4

CashOnDelivery.php in payment/uc_payment_pack/src/Plugin/Ubercart/PaymentMethod/CashOnDelivery.php

Check.php in payment/uc_payment_pack/src/Plugin/Ubercart/PaymentMethod/Check.php

CreditCardPaymentMethodBase.php in payment/uc_credit/src/CreditCardPaymentMethodBase.php

FreeOrder.php in payment/uc_payment/src/Plugin/Ubercart/PaymentMethod/FreeOrder.php

Other.php in payment/uc_payment_pack/src/Plugin/Ubercart/PaymentMethod/Other.php

PayPalPaymentMethodPluginBase.php in payment/uc_paypal/src/Plugin/Ubercart/PaymentMethod/PayPalPaymentMethodPluginBase.php

TwoCheckout.php in payment/uc_2checkout/src/Plugin/Ubercart/PaymentMethod/TwoCheckout.php