code: protected function validateInvoiceAddressOptIn()
      {
          $return = true;
          $optin = (int) $this->getRequestParameter('oegdproptin_invoiceaddress');
          $changeExistigAddress = (int) $this->getRequestParameter('oegdproptin_changeInvAddress');
  
          if (\OxidEsales\Eshop\Core\Registry::getConfig()->getConfigParam('blOeGdprOptinInvoiceAddress')
              && (1 == $changeExistigAddress)
              && (1 !== $optin)
          ) {
              $return = false;
          }
          return $return;
      }
docs: Validate invoice address optin.
  Needed if user changes invoice address.
  
  @return bool