This snippet moves ordered items in draft state if they are in specific category when order is marked complete. Same without category filter: https://gist.github.com/Pikkulahti/545e5f738027c6e36637f49c6ccd2743

draft_complete_orders_in_category.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();
    foreach( $items as $product ) {
        // Get product categories
        $terms = get_the_terms( $product['product_id'], 'product_cat' );
        if( !empty($terms) ) {
            // Array to store slugs into
            $slugs = array();
            // Take slugs out of WP_Term objects
            foreach ($terms as $category) {
                $slugs[] = $category->slug;
            }
            
            // Category that product must belong to
            $filter_category = 'shirts';
            // Loop through slugs
            foreach ( $slugs as $slug ) {
                // Check if product category matches the filter
                if ( $filter_category === $slug ) {
                    // Change product status to draft
                    wp_update_post( 
                        array(
                            'ID' => $product['product_id'],
                            'post_status' => 'draft',
                        ) 
                    );
                    break;
                }
            }
        }
    }
}
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 payment methods.