if with custom product attributes

I am trying to adapt the following code to work with custom attributes (and not with global attributes (pa_) but can’t get it to work. This one is working fine, it’s the unaltered code:

add_filter( 'woocommerce_coupon_is_valid', 'check_if_coupons_are_valid', 10, 3 );
function check_if_coupons_are_valid( $is_valid, $coupon, $discount ){
    // YOUR ATTRIBUTE SETTINGS BELOW:
    $taxonomy   = 'pa_xyz';
    $term_slugs = array('zyx');

    // Loop through cart items and check for backordered items
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        foreach( $cart_item['variation'] as $attribute => $term_slug ) {
            if( $attribute === 'attribute_'.$taxonomy && in_array( $term_slug, $term_slugs ) ) {
                $is_valid = false; // attribute found, coupons are not valid
                break; // Stop and exit from the loop
            }
        }
    }
    return $is_valid;
}

So something like if $product_attribute[‘value’] is xxx… any help is really appreciated!