Backlinks with bundle class and Twig: Editorship bundle class providing a getter for all series nodes referencing the editorship entity, and Twig Template using this getter to render the series nodes below the editorship as backlinks. Uses the Twig Tweak module!

EditorshipBundle.php

<?php

namespace Drupal\frs_entities\Entity\Bundle;

use Drupal\Core\Field\FieldItemList;

/**
 * A bundle class for node entities.
 *
 * @property FieldItemList $field_content
 * @property FieldItemList $field_teaser
 */
class EditorshipBundle extends NodeBundle {

  /**
   * Find all series entities referencing this editorship.
   *
   * @return array
   *   The series nodes referencing this editorship, if any.
   */
  public function getSeries(): array {

    // Find the ids of all series entities referencing this editorship.
    $ids = \Drupal::entityQuery('node')
      ->condition('type', 'series')
      ->condition('field_editorship', $this->id())
      ->condition('status', 1)
      ->condition('field_discontinued', TRUE, '<>')
      ->sort('title')
      ->accessCheck(FALSE)
      ->execute();
    // Return the loaded entities, or an empty array if none were found.
    return empty($ids) ? [] : self::loadMultiple($ids);

  }
}

node--editorship.html.twig

{{ include('node.html.twig') }}
{{ {'#cache': {tags: ['node_list:series']}} }}
{% set series = node.getSeries() %}
{% if series is not empty %}
  <h2>Series of this editorship</h2>
  {% for series in node.getSeries() %}
    <article>
      <h3>{{ series|entity_link }}</h3>
      <p>{{ series.field_teaser|view }}</p>
    </article>
  {% endfor %}
{% endif %}

There's a solution:
It's possible to add custom cache tags in a twig template, this way:
{{ {'#cache': {tags: ['node_list:series']}} }}
Now the cache will be invalidated whenever a series node is added, changed or deleted!