Wordpress - Show posts without term

You cannot use the same object of WP_Query twice. Therefore you need to create another one with a tax_query parameter to fetch posts which are not assigned to any term.

//fetch all reviews which have no assigned term in 'review-product'
$taxonomy  = 'review-product';
$post_type = 'reviews';
$args = [
    'post_type' => $post_type,
    'tax_query' => [
        [
            'taxonomy' => $taxonomy,
            'terms'    => get_terms( $taxonomy, [ 'fields' => 'ids'  ] ),
            'operator' => 'NOT IN'
        ]
    ]
];

$query = new \WP_Query( $args );

The idea is to fetch a list of all terms of your taxonomy and pass them as argument to your tax-query with the NOT IN operator.

The second loop in your example should walk over the new WP_Query object.

$posts_query = new WP_Query( array(
    'tax_query' => array(
        array(
            'taxonomy' => 'foo',
            'operator' => 'NOT EXISTS', // or 'EXISTS'
        ),
    ),
) );

see: https://core.trac.wordpress.org/ticket/29181