Undeletable WordPress pages

undeletable-wordpress-pages.php

<?php

function restrict_post_deletion ($post_ID) {
  $restricted_pages = [
    get_option('page_on_front'),
    123,
    666
  ];

  if (!current_user_can('administrator') && in_array($post_ID, $restricted_pages)) {
    echo "You dont have permission to delete this page.";
    exit;
  }
}

add_action('wp_trash_post', 'restrict_post_deletion', 10, 1);
add_action('before_delete_post', 'restrict_post_deletion', 10, 1);