Seu uso da API do gerenciador está incorreto.
$m = new MongoDB\Driver\Manager("mongodb://localhost:27017");
$filter= array('email' => $email);
$options = array(
'limit' => 1
);
$query = new MongoDB\Driver\Query($filter, $options);
$rows = $m->executeQuery('clients.suscriptions', $query);
Alternativamente, você deve instalar a biblioteca através do composer que fornece sintaxe semelhante à API antiga.
require 'vendor/autoload.php';
$m= new MongoDB\Client("mongodb://127.0.0.1/");
$db = $m->clients;
$collection = $db->suscriptions;
$query = array('email' => $email);
$document = $collection->findOne($query);
https://docs.mongodb.com/ php-library/master/tutorial/crud/#find-one-document
