Do you mean RDF/RSS 1.0 feed?

It is not deprecated and should be available just fine at `/feed/rdf`

To force RSS 2.0 links to use RSS 0.92 template try this:

    remove_filter( 'do_feed_rss2', 'do_feed_rss2' );
    add_filter( 'do_feed_rss2', 'do_feed_rss' );