code: private function isValueInList(Cell $cell)
      {
          $cellValue = $cell->getValue();
          $dataValidation = $cell->getDataValidation();
  
          $formula1 = $dataValidation->getFormula1();
          if (!empty($formula1)) {
              // inline values list
              if ($formula1[0] === '"') {
                  return in_array(strtolower($cellValue), explode(',', strtolower(trim($formula1, '"'))), true);
              } elseif (strpos($formula1, ':') > 0) {
                  // values list cells
                  $matchFormula = '=MATCH(' . $cell->getCoordinate() . ', ' . $formula1 . ', 0)';
                  $calculation = Calculation::getInstance($cell->getWorksheet()->getParent());
  
                  try {
                      $result = $calculation->calculateFormula($matchFormula, $cell->getCoordinate(), $cell);
  
                      return $result !== Functions::NA();
                  } catch (Exception $ex) {
                      return false;
                  }
              }
          }
  
          return true;
      }
docs: Does this cell contain valid value, based on list?
  
  @param Cell $cell Cell to check the value
  
  @return bool