Custom taxonomy in custom post RSS feed

You can hook into the request filter and set the post types to whatever you wish:

add_filter( 'request', 'cpt_in_rss' );

function cpt_in_rss( $request ) {

    if ( isset( $request['feed'] ) && ! isset( $request['post_type'] ) ) {
        $request['post_type'] = [ 'post', 'cpt1', 'cpt2' ];
    }

    return $request;

}

Here’s a trac ticket about what you are asking.