code: protected function getSoftDeleteField(Model $record)
      {
          if ($field = $this->softDeleteField()) {
              return $field;
          }
  
          $key = $this->getSoftDeleteKey($record);
  
          return Str::dasherize($key);
      }
docs: Get the JSON API field that is used for the soft-delete value.
  
  @param Model $record
  @return string