Issue #3305413 by joachim, Anchal_gupta, rpayanm, smustgrave, quietone: change...

    // Create two taxonomy terms.

    $term1 = $this->createTerm($this->vocabulary);

    // RSS display must be added manually.

    $this->drupalGet("admin/structure/types/manage/article/display");

    $edit = [

      "display_modes_custom[rss]" => '1',

    ];

    $this->submitForm($edit, 'Save');

    // Add the RSS display.

    $default_display = $this->container->get('entity_display.repository')->getViewDisplay('node', 'article');

    $rss_display = $default_display->createCopy('rss');

    $rss_display->save();

    // Change the format to 'RSS category'.

    $this->drupalGet("admin/structure/types/manage/article/display/rss");

    $edit = [

      "fields[taxonomy_" . $this->vocabulary->id() . "][type]" => 'entity_reference_rss_category',

      "fields[taxonomy_" . $this->vocabulary->id() . "][region]" => 'content',

    ];

    $this->submitForm($edit, 'Save');

    // Post an article.

    $edit = [];

    $edit['title[0][value]'] = $this->randomMachineName();

    $edit[$this->fieldName . '[]'] = $term1->id();

    $this->drupalGet('node/add/article');

    $this->submitForm($edit, 'Save');

    $rss_display->setComponent('taxonomy_' . $this->vocabulary->id(), [

      'type' => 'entity_reference_rss_category',

      'region' => 'content',

    ]);

    $rss_display->save();

    // Create an article.

    $node = $this->drupalCreateNode([

      'type' => 'article',

      $this->fieldName => $term1->id(),

    ]);

    // Check that the term is displayed when the RSS feed is viewed.

    $this->drupalGet('rss.xml');

    $view->getDisplay()->overrideOption('arguments', $arguments);

    $view->storage->save();

    // Check the article is shown in the feed.

    $node = $this->drupalGetNodeByTitle($edit['title[0][value]']);

    $raw_xml = '<title>' . $node->label() . '</title>';

    $this->drupalGet('taxonomy/term/all/feed');

    $this->assertSession()->responseContains($raw_xml);