Add and use a new stock status in WooCommerce

functions.php

<?php
/**
 * This code adds a lowstock stock status to WooCommerce products.  There is more code
 * included here for examples of usage, such as hiding the lowstock status products from
 * queries so they aren't displayed in shop pages,add badges to products, and update the
 * availability messages on product pages.
 *
 * Please see this WordPress Stack Exchange question for more information and background:
 *
 * https://wordpress.stackexchange.com/questions/348759/exclude-products-with-a-stock-lower-than-the-low-stock-threshold-from-the-shop-l
 */

/**
 * Add low stock to WooCommerce stock status options.
 */
function add_low_stock_option( $options ) {
	$options['lowstock'] = __( 'Low Stock', 'textdomain' );
	return $options;
}

add_filter( 'woocommerce_product_stock_status_options', 'add_low_stock_option' );

/**
 * Save the low stock status.
 */
function save_low_stock_status( $product_id ) {
	$product = wc_get_product( $product_id );

	// Stock managed for product individually and between "Out of Stock" and "Low Stock Amount".
	if ( $product->get_manage_stock() &&
		( $product->get_stock_quantity() <= $product->get_low_stock_amount() )
	) {
		// Update to our low stock status.
		update_post_meta( $product_id, '_stock_status', 'lowstock' );
	}
}

add_action( 'woocommerce_process_product_meta', 'save_low_stock_status', 20 );

/**
 * Get availablity status.
 */
function get_custom_availability( $data, $product ) {
	$stock_status = get_post_meta( $product->id , '_stock_status' , true );
	if ( 'lowstock' === $stock_status ) {
		$data = [
			'availability' => __( 'Low stock', 'textdomain' ),
			'class' => 'low-stock'
		];
	}

	return $data;
}

add_action( 'woocommerce_get_availability', 'get_custom_availability', 20, 2 );

/**
 * Hide the lowstock stock status products in queries.
 */
function hide_low_stock_products( $q ) {
	$meta_query = $q->get( 'meta_query' );

	$meta_query[] = [
		'key' => '_stock_status',
		'value' => 'lowstock',
		'compare' => '!=',
	];

	$q->set( 'meta_query', $meta_query );
}

add_action( 'woocommerce_product_query', 'hide_low_stock_products' );

/**
 * Add a low stock badge on shop pages.
 */
function add_low_stock_badge() {
	global $product;

	$stock_status = $product->get_stock_status();

	if ( 'lowstock' === $stock_status ) {
		echo '<span class="low-stock-badge">' . __( 'Low stock', 'textdomain' ) . '</span>';
	}
}

add_action( 'woocommerce_before_shop_loop_item', 'add_low_stock_badge' );

/**
 * Update all product stock statuses that meet the criteria.
 *
 * NOTE: Only run this to update your products if the query below
 * satisfies your desired results.  Especially if running on an active site.
 */
function update_product_stock_status(){
	global $wpdb;

	// Low Quantity threshold.
	$threshold = '2';

	// SQL statement to update _stock_status to lowstock on all products where _stock >= 1 AND _stock <= $threshold set for lowstock status.
	$sql .= "UPDATE $wpdb->postmeta stock, (SELECT DISTINCT post_id FROM $wpdb->postmeta WHERE meta_key = '_stock' AND meta_value BETWEEN 1 AND $threshold ) id SET stock.meta_value = 'lowstock' WHERE stock.post_id = id.post_id AND stock.meta_key = '_stock_status';";

	// run queries
	$wpdb->query( $sql );
}

Line 44 - $product->id
This needs to be updated to

$product->get_id()

Else you'll get a notice that ID is called incorrectly within WooCommerce

I want to add SoldOUT status.
but I don't know how to set out of stock and when to use in_stock must return false.
can you help me?