Original code from

function html_widget_title( $title ) {
    //HTML tag opening/closing brackets
    $title = str_replace( '[', '<', $title );
    $title = str_replace( '[/', '</', $title );

    //<strong></strong>
    $title = str_replace( 's]', 'strong>', $title );
    //<em></em>
    $title = str_replace( 'e]', 'em>', $title );

    return $title;
}
add_filter( 'widget_title', 'html_widget_title' );

Amended code for line breaks only

function html_widget_title( $title ) {
    //HTML tag opening/closing brackets
    $title = str_replace( '[', '<', $title );
    $title = str_replace( 'br]', 'br>', $title );
    return $title;
}
add_filter( 'widget_title', 'html_widget_title' );