class ForumListingBreadcrumbBuilder

class ForumListingBreadcrumbBuilder extends ForumBreadcrumbBuilderBase {
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return $route_match->getRouteName() == 'forum.page' && $route_match->getParameter('taxonomy_term');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build(RouteMatchInterface $route_match) {
        $breadcrumb = parent::build($route_match);
        $breadcrumb->addCacheContexts([
            'route',
        ]);
        // Add all parent forums to breadcrumbs.
        
        /** @var \Drupal\taxonomy\TermInterface $term */
        $term = $route_match->getParameter('taxonomy_term');
        $term_id = $term->id();
        $breadcrumb->addCacheableDependency($term);
        $parents = $this->termStorage
            ->loadAllParents($term_id);
        if ($parents) {
            foreach (array_reverse($parents) as $parent) {
                if ($parent->id() != $term_id) {
                    $breadcrumb->addCacheableDependency($parent);
                    $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page', [
                        'taxonomy_term' => $parent->id(),
                    ]));
                }
            }
        }
        return $breadcrumb;
    }

}