No longer possible to customize the widget

Proposed the removal of final class.

Can you share the widget code?

Here it is the code for the widget:

namespace Drupal\my_project\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\double_field\Plugin\Field\FieldWidget\DoubleField;

/**
 * Custom plugin implementation of the 'double_field' widget.
 *
 * @FieldWidget(
 *   id = "custom_double_field",
 *   label = @Translation("Custom Double Field"),
 *   field_types = {"double_field"}
 * )
 */
class CustomDoubleField extends DoubleField {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state): array {
    $form_element = parent::formElement($items, $delta, $element, $form, $form_state);
    $field_settings = $this->getFieldSettings();
    $settings = $this->getSettings();

    // Create custom form element using Flatpickr as field datepicker.
    foreach (['first', 'second'] as $subfield) {
      $widget_type = $settings[$subfield]['type'];
      if ($widget_type == 'datetime') {
        if ($field_settings['storage'][$subfield]['datetime_type'] == 'date') {
          $name = 'open_position-' . $subfield;
          $default_value = $items[$delta]->createDate($subfield);

          $form_element[$subfield]['#attributes']['flatpickr-name'] = $name;
          $form_element[$subfield]['#attributes']['class'][] = 'field--widget-datetime-flatpickr';
          $form_element[$subfield]['#attached']['library'][] = 'datetime_flatpickr/flatpickr-init';
          $form_element[$subfield]['#attached']['drupalSettings']['datetimeFlatPickr'][$name] = [
            'settings' => [
              'altInput' => TRUE,
              'altFormat' => 'd/m/Y',
              'dateFormat' => 'Y-m-d',
              'defaultDate' => !empty($default_value) ? $default_value->format('Y-m-d') : date('Y-m-d'),
              'position' => 'auto',
              'disabledWeekDays' => [],
            ],
          ];
        }
        else {
          if ($form_element[$subfield]['#default_value']) {
            $form_element[$subfield]['#default_value']->setTimezone(new \DateTimezone(date_default_timezone_get()));
          }
          // Ensure that the datetime field processing doesn't set its own time
          // zone here.
          $form_element[$subfield]['#date_timezone'] = date_default_timezone_get();
        }
      }
    }

    return $form_element;
  }

  /**
   * {@inheritdoc}
   */
  protected function getSubwidgets(string $subfield_type, bool $list): array {
    $subwidgets = parent::getSubwidgets($subfield_type, $list);

    if ($subfield_type == 'datetime_iso8601') {
      $subwidgets['datetime'] = $this->t('Flatpickr Datepicker');
    }

    return $subwidgets;
  }

}

Generally plugin classes are not part of public API.
https://www.drupal.org/about/core/policies/core-change-policies/bc-polic... →

In your case it should be simpler to implement hook_field_widget_single_element_WIDGET_TYPE_form_alter instead of creating new widget.

Thanks for your suggestion. We are going to rewrite the widget using hook_field_widget_single_element_WIDGET_TYPE_form_alter. Very appreciated