The Validation::beforeSavePaymentInformationAndPlaceOrder() method can be simplified by removing unused $billingAddress argument

This can be simplified to:

/**
 * @param \Magento\Checkout\Api\PaymentInformationManagementInterface $subject
 * @param int $cartId
 * @param \Magento\Quote\Api\Data\PaymentInterface $paymentMethod
 * @return void
 * @SuppressWarnings(PHPMD.UnusedFormalParameter)
 */
public function beforeSavePaymentInformationAndPlaceOrder(
	\Magento\Checkout\Api\PaymentInformationManagementInterface $subject,
	$cartId,
	\Magento\Quote\Api\Data\PaymentInterface $paymentMethod
) {
	if ($this->isAgreementEnabled()) {
		$this->validateAgreements($paymentMethod->getExtensionAttributes()->getAgreementIds());
	}
}

A trick: a Ā«beforeĀ» plugin can avoid declaration of unused last arguments if it needs not modify any argument