This snippet moves ordered items in draft state when order is marked complete. Same with category filter: https://gist.github.com/Pikkulahti/2504f6539b67efe417bd9b1382f72658

draft_complete_orders.php

<?php
/**
 * Moves ordered items in draft state.
 */
function custom_draft_complete_orders( $order_id ) {
    // Fetch order infromation
    $order = new WC_Order( $order_id );
    // Get products on order
    $items =  $order->get_items();
  
  // Loop through products
    foreach ( $items as $product ) {
        // Change status to draft
        wp_update_post( 
            array(
                'ID' => $product['product_id'],
                'post_status' => 'draft',
            ) 
        );
    }
}
add_action( 'woocommerce_order_status_completed', 'custom_draft_complete_orders');

the woocommerce_order_status_completedhook only works for certain payment methods, so you need to figure out the right hooks accoring to your requirements.