Shopware 6 - Strict Category Listing

<?php
declare(strict_types=1);

namespace Foo\Bar\Subscriber;

use Shopware\Core\Framework\DataAbstractionLayer\Search\Filter\EqualsFilter;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Shopware\Core\Content\Product\Events\ProductListingCriteriaEvent;

/**
 * @class FrontendSubscriber
 * @package Foo\Bar\Subscriber
 */
class FrontendSubscriber implements EventSubscriberInterface
{

    /**
     * @return string[]
     */
    public static function getSubscribedEvents(): array
    {
        return [
            ProductListingCriteriaEvent::class => 'onLoadProductListing'
        ];
    }

    /**
     * @param ProductListingCriteriaEvent $event
     * @return void
     */
    public function onLoadProductListing(ProductListingCriteriaEvent $event): void
    {
        $request = $event->getRequest();
        $criteria = $event->getCriteria();

        $categoryId = $request->get('navigationId');

        $criteria->addFilter(
            new EqualsFilter('categories.id', $categoryId)
        );
    }
}