Custom taxonomy and query multi conditions

Let’s say you have 5 instances:
200, 201, 202, 203, 204

NOT IN {200, 201, 202} is equal to IN {203, 204} (all the other instances).

So you don’t need your IN operator anymore because its results are already included from using the NOT IN operator.

In your case, this logic works assuming an article does not have multiple locations.

$query->set('tax_query',

        array(
            array (
                'taxonomy' => 'location',
                'operator' => 'NOT IN',
                'field' => 'slug',
                'terms' => $notavailableRegions,
            )
        )
);