View Private Published Page with URL Code (no login required)

You can actually hook the built-in page/post password feature and set it up to check for url params that match page/post password, and then change the value of $required to false.

add_filter( 'post_password_required', 'my_allow_url_param', 10, 2 );

function my_allow_url_param( $required, $post ) {

    if ( empty( $post->post_password ) ) {
        $required = false;
         return $required;
    }

    if ( $_GET['code'] == $post->post_password ) {
       $required = false;
    }
    return $required;
}

If you had a page with the slug of /sample-page/ and in the admin set a password of Cf5aTgZqBz5h6g1y858eQmpTWizzHzZN in the visibility setting on edit screen of that page, you could access it without logging in or typing a password with the following url:

http://sitename.com/sample-page/?code=Cf5aTgZqBz5h6g1y858eQmpTWizzHzZN