Create own custom CKEditor filter

LinkTitleFilter.php

<?php

namespace Drupal\MODULE_NAME\Plugin\Filter;

use Drupal\Component\Utility\Html;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * Filter Link Title attribute.
 *
 * @Filter(
 *   id = "custom_link_title_html",
 *   title = @Translation("Link Title Filter"),
 *   description = @Translation("Add text (new window) to links in content."),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_REVERSIBLE,
 * )
 */
class LinkTitleFilter extends FilterBase {

  /**
   * Performs the filter processing.
   *
   * @param string $text
   *   The text string to be filtered.
   * @param string $langcode
   *   The language code of the text to be filtered.
   *
   * @return \Drupal\filter\FilterProcessResult
   *   The filtered text, wrapped in a FilterProcessResult object, and possibly
   *   with associated assets, cacheability metadata and placeholders.
   *
   * @see \Drupal\filter\FilterProcessResult
   */
  public function process($text, $langcode) {
    $html = Html::load($text);
    $links = $html->getElementsByTagName('a');

    /** @var \DOMElement $link */
    foreach ($links as $link) {
      $target_attribute = $link->getAttribute('target');
      $title_name = $link->nodeValue;

      if ($target_attribute === '_blank') {
        $link->setAttribute('title', $title_name . " " . $this->t("(new window)"));
      }
    }
    $text = Html::serialize($html);

    return new FilterProcessResult($text);
  }

}