The <channel> element created by views RSS is no longer rendered during preprocessing

In versions prior to 8.8.0-alpha1 template_preprocess_views_view_rss() has the following line:

  $variables['channel_elements'] = \Drupal::service('renderer')->render($style->channel_elements);

The channel elements are then printed in views-view-rss.html.twig:

<rss version="2.0" xml:base="{{ link }}"{{ namespaces }}>
  <channel>
    <title>{{ title }}</title>
    <link>{{ link }}</link>
    <description>{{ description }}</description>
    <language>{{ langcode }}</language>
    {{ channel_elements }}
    {{ items }}
  </channel>
</rss>

By forcing the {{ channel_elements }} to render early, its contents could no longer be changed by other modules or themers. The Views RSS module for example previously extended the channel elements with more elements from the RSS spec: http://www.rssboard.org/rss-profile and this required re-rendering.

From 8.8.0-alpha1 this early rendering of RSS channel elements in template_preprocess_views_view_rss() has been removed so other modules can add channel elements without having to re-render.