不管怎样，我只是放弃了“\\u not\\u in”方法，并采用了以下有点鬼祟祟的解决方案：

//Create the exclusion rule
$new_tax_query = array(
    array(
        \'taxonomy\' => \'x_category\',
        \'field\'    => \'slug\',
        \'terms\'    => array(\'y-slug\'),
        \'operator\' => \'NOT IN\',
    )
);

//If there is already a tax_query, \'AND\' our new rule with the entire existing rule.
$tax_query = $query->get(\'tax_query\');
if(!empty($tax_query)) {
    $new_tax_query = array(
        \'relation\' => \'AND\',
        $tax_query,
        $new_tax_query
    );              
}

// var_dump($new_tax_query);

$query->set(\'tax_query\', $new_tax_query);