How to hide products if out of stock (if variable product – if all variation are out of stock) in Woocommerce?

You can use the woocommerce_product_query action hook to remove a product if all variation is outofstock. code will go in your active theme functions.php file.

function hide_product_if_all_variation_and_simple_product_is_outofstock( $q ) {

    global $wpdb;

    // Get ids of products which you want to hide
    $array_of_product_id = array();

    // get all variable product Ids
    $variable_products_ids = wc_get_products( array(
        'limit'  => -1,
        'status' => 'publish',
        'type'   => 'variable',
        'return' => 'ids',
    ));

    // Loop through variable products
    foreach( $variable_products_ids as $variable_id ) {
        $count = $wpdb->get_var( $wpdb->prepare( "
            SELECT COUNT(ID)
            FROM {$wpdb->posts} p
            INNER JOIN {$wpdb->postmeta} pm
                ON p.ID           =  pm.post_id
            WHERE p.post_type     =  'product_variation'
                AND p.post_status =  'publish'
                AND p.post_parent =  %d
                AND pm.meta_key   =  '_stock_status'
                AND pm.meta_value != 'outofstock'
        ", $variable_id ) );

        $count = $count > 0 ? true : false;         

        if( !$count ){
            $array_of_product_id[] = $variable_id;
        }
    }

    // get all simple product Ids
    $simple_products_ids = wc_get_products( array(
        'limit'  => -1,
        'status' => 'publish',
        'type'   => 'simple',
        'return' => 'ids',
    ));

    // Loop through variable products
    foreach( $simple_products_ids as $simple_id ) {
        $count = $wpdb->get_var( $wpdb->prepare( "
            SELECT COUNT(ID)
            FROM {$wpdb->posts} p
            INNER JOIN {$wpdb->postmeta} pm
                ON p.ID           =  pm.post_id
            WHERE p.post_type     =  'product'
                AND p.post_status =  'publish'
                AND p.ID =  %d
                AND pm.meta_key   =  '_stock_status'
                AND pm.meta_value != 'outofstock'
        ", $simple_id ) );
        
        $count = $count > 0 ? true : false;         

        if( !$count ){
            $array_of_product_id[] = $simple_id;
        }
    }

    if( !empty( $array_of_product_id ) ){
        $q->set( 'post__not_in', $array_of_product_id );
    }

}
add_action( 'woocommerce_product_query', 'hide_product_if_all_variation_and_simple_product_is_outofstock' );