code: public function beforeDeleteById(
          \Magento\Customer\Api\CustomerRepositoryInterface $subject,
          $customerId
      ) {
          $this->deleteDwnl($customerId);
          $result = [$customerId];
          return $result;
      }
docs: Remove customer related data from Bonus Hybrid tables on customer delete from adminhtml.
  
  @param \Magento\Customer\Api\CustomerRepositoryInterface $subject
  @param $customerId
  @return array