That sounds a feature request, probably better to raise the request on github. I would say for now if you really need it, maybe you can get the latest revision date and inject it in the api response, something like below may work:
$app->on('singleton.getData.after.<singletonname>', function ($singleton, &$data) use ($app) {
$revisions = $app->helper('revisions')->getList($singleton['_id']);
if ($revisions) {
$data['_modified'] = $revisions[0]['_created'];
}
});