Is there advanced configuration for CKEditor in Drupal 8?
In regards to "allowedContent", this is set to true if you don't check "Limit allowed HTML tags and correct faulty HTML"( or another filter of type of FilterInterface::TYPE_HTML_RESTRICTOR) on your text format, otherwise it outputs the tags you have added there.



For more info on this, you can view:

ckeditor/src/Plugin/CKEditorPlugin/Internal:getConfig()

and 

ckeditor/src/Plugin/CKEditorPlugin/Internal:generateACFSettings()

If wanted to override the Internal:getConfig method, you could create a new class that extends it and then replace the class using hook_ckeditor_plugin_info_alter().

function mymodule_ckeditor_plugin_info_alter(array &amp;$plugins) {
  $plugins['internal']['class'] = 'Drupal\mymodule\Plugins\CKEditorPlugin\CustomInternal';
}


And then your custom override class:

&lt;?php

namespace Drupal\mymodule\Plugins\CKEditorPlugin;

use Drupal\ckeditor\Plugin\CKEditorPlugin\Internal;
use Drupal\editor\Entity\Editor;

/**
 * Allow custom config settings.
 *
 * @CKEditorPlugin(
 *   id = "custom_internal",
 *   label = @Translation("Custom CKEditor core")
 * )
 */
class CustomInternal extends Internal {

  /**
   * {@inheritdoc}
   */
  public function getConfig(Editor $editor) {

    $config = parent::getConfig($editor);

    // Put your custom configs here.
    $config['allowedContent'] = TRUE;

    return $config;
  }

}