Detect RSS endpoint

function pm_extra_rss_endpoint($endpoints) {
    return "{$endpoints}|rss";
}
add_filter('permalink_manager_endpoints', 'pm_extra_rss_endpoint', 9);

function pm_fix_rss_query($query, $old_query, $uri_parts, $pm_query, $content_type) {
    if(isset($query['rss']) && isset($query['do_not_redirect'])) {
        unset($query['rss']);
        unset($query['do_not_redirect']);

        $query['feed'] = 'rss';
    }

    return $query;
}
add_filter('permalink_manager_filter_query', 'pm_fix_rss_query', 3, 5);