Steps to reproduce the issue

5.3 beta 1, unpublish the Backward Compatibility Plugin and open a page that does not exist

Expected result

404 page is thrown

Actual result

Class JRouterSite not found.

Setting a tainted URL for an article still call:

public static function getInstance($client, $options = [])
    {
        if (empty(self::$instances[$client])) {
            // Create a Router object
            $classname = 'JRouter' . ucfirst($client);

            if (!class_exists($classname)) {
                throw new \RuntimeException(Text::sprintf('JLIB_APPLICATION_ERROR_ROUTER_LOAD', $client), 500);
            }

            // Check for a possible service from the container otherwise manually instantiate the class
            if (Factory::getContainer()->has($classname)) {
                self::$instances[$client] = Factory::getContainer()->get($classname);
            } else {
                self::$instances[$client] = new $classname();
            }
        }

        return self::$instances[$client];
    }

The class JRouterSite is not found without the Backward Compatibility Plugin enabled, so an exception is thrown.