My sample code is:

public class UndertowApp {
    public static void main(String[] args) {
        Undertow server = Undertow.builder()
            .addHttpListener(8080, "0.0.0.0")
                .setHandler(exchange -> {
                    exchange.setProtocol(new HttpString("HTTP/1.0"));
                    exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
                    exchange.getResponseSender().send("Hello from Undertow!");
                }).build();

        server.start();
        System.out.println("Server started on http://0.0.0.0:8080");
{{    }}}

}

Despite setting the protocol to "HTTP/1.0", the status line says 'HTTP/1.1".

I think it would be better if the protocol field from the HttpServerExchange were sent back to the client, not a hard-coded string.