Show Additional Field Name

Version 2.9.0 makes this pretty easy

Using the “simple_links_additional_fields_output” filter like so should do the trick

add_filter( 'simple_links_additional_fields_output', 'sl_show_column_names', 1, 3 );
function sl_show_column_names( $output, $fields, $class ){
	$additional_fields = null;
	$format = ' %s %s : <span class="%s">%s</span>';

	foreach( $class->args[ 'fields' ] as $field ){
		$data = $class->getAdditionalField( $field );
		if( ! empty( $data ) ){
			$additional_fields .= sprintf( $format,
				$class->args[ 'separator' ],
				$field,
				str_replace( ' ', '-', strtolower( $field ) ),
				$data
			);
		}
	}
	return $additional_fields;
}