TestFieldNoSettingsFormatter.php

<?php

namespace Drupal\field_test\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;

/**
 * Plugin implementation of the 'field_no_settings' formatter.
 *
 * @FieldFormatter(
 *   id = "field_no_settings",
 *   label = @Translation("Field no settings"),
 *   field_types = {
 *     "test_field",
 *   },
 *   weight = -10
 * )
 */
class TestFieldNoSettingsFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            // This formatter only needs to output raw for testing.
            $elements[$delta] = [
                '#markup' => $item->value,
            ];
        }
        return $elements;
    }

}