taxonomy_manager-2.0.3/modules/taxonomy_manager_merge/taxonomy_manager_merge.module

<?php

/**
 * @file
 * Taxonomy Manager Merge.
 */

use Drupal\Core\Form\FormStateInterface;
use Drupal\taxonomy\VocabularyInterface;
use Drupal\taxonomy_manager_merge\Form\MergeTermsForm;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function taxonomy_manager_merge_form_taxonomy_manager_vocabulary_terms_form_alter(array &$form, FormStateInterface $form_state, string $form_id): void {
  if (!isset($form['voc']['#value']) || !($form['voc']['#value'] instanceof VocabularyInterface)) {
    return;
  }

  $taxonomy_vocabulary = $form['voc']['#value'];

  $form['toolbar']['merge'] = [
    '#type' => 'submit',
    '#name' => 'merge',
    '#value' => t('Merge terms'),
    '#ajax' => [
      'callback' => [MergeTermsForm::class, 'mergeTermsFormCallback'],
    ],
    '#attributes' => [
      'disabled' => TRUE,
    ],
    '#access' => \Drupal::currentUser()->hasPermission('edit terms in ' . $taxonomy_vocabulary->id()),
  ];

  // Attach custom library.
  $form['#attached']['library'][] = 'taxonomy_manager_merge/tree';
}