get_author_feed_link( int $author_id, string $feed = '' )

function get_author_feed_link( $author_id, $feed = '' ) {
	$author_id = (int) $author_id;
	$permalink_structure = get_option('permalink_structure');

	if ( empty($feed) )
		$feed = get_default_feed();

	if ( '' == $permalink_structure ) {
		$link = home_url("?feed=$feed&author=" . $author_id);
	} else {
		$link = get_author_posts_url($author_id);
		if ( $feed == get_default_feed() )
			$feed_link = 'feed';
		else
			$feed_link = "feed/$feed";

		$link = trailingslashit($link) . user_trailingslashit($feed_link, 'feed');
	}

	/**
	 * Filters the feed link for a given author.
	 *
	 * @since WP-1.5.1
	 *
	 * @param string $link The author feed link.
	 * @param string $feed Feed type.
	 */
	$link = apply_filters( 'author_feed_link', $link, $feed );

	return $link;
}