Change the title tag of WordPress

functions.php

<?php
/**
 * Custom title tag in WordPress
 *
 * @author Kevin Pliester
 */

/**
 * Set custom title
 *
 * @param string $title default title.
 *
 * @return array
 */
function custom_title( $title ) {
  return $title;
}

/**
 * Init custom title
 */
add_filter( 'pre_get_document_title', [ $this, 'custom_title' ], PHP_INT_MAX, 2 );