TextFieldItemList.php

<?php

namespace Drupal\text\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldItemList;
use Drupal\Core\Form\FormStateInterface;

/**
 * Defines an item list class for text fields.
 */
class TextFieldItemList extends FieldItemList {
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesFormValidate(array $element, array &$form, FormStateInterface $form_state) {
        if ($allowed_formats = $this->getSetting('allowed_formats')) {
            $field_name = $this->definition
                ->getName();
            $submitted_values = $form_state->getValue([
                'default_value_input',
                $field_name,
            ]);
            foreach ($submitted_values as $delta => $value) {
                if (!in_array($value['format'], $allowed_formats, TRUE)) {
                    $form_state->setErrorByName("default_value_input][{$field_name}][{$delta}][format", $this->t("The selected text format is not allowed."));
                }
            }
        }
        parent::defaultValuesFormValidate($element, $form, $form_state);
    }

}