woo-set-backorder-max.php

add_filter( 'woocommerce_product_get_stock_status', 'my_plugin_set_stock_status', 10, 2 );
add_filter( 'woocommerce_product_variation_get_stock_status', 'my_plugin_set_stock_status', 10, 2 );
/**
 * Set maximum back orders.
 *
 * @param $stock_status string product stock status.
 * @param $product integer product.
 *
 * @return mixed|string
 */
function my_plugin_set_stock_status( $stock_status, $product ) {

	$numleft  = $product->get_stock_quantity();

	if($numleft <= -200) {
		// out of stock
		return is_admin() ? $stock_status : 'outofstock';
	}
	else if( -200 < $numleft && $numleft <= 0 ) {
		return is_admin() ? $stock_status : 'onbackorder';
	}
	else {
		return is_admin() ? $stock_status : 'instock';
	}
}

add_action( 'woocommerce_checkout_before_order_review', 'my_plugin_out_of_stock_items' );
add_action( 'woocommerce_before_cart', 'my_plugin_out_of_stock_items' );
/**
 * Remove out of stock items from cart.
 */
function my_plugin_out_of_stock_items() {

	if ( WC()->cart->is_empty() ) {
		return;
	}

	$removed_products = [];

	// Loop through cart items
	foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
		// Get an instance of the WC_Product Object
		$product = $cart_item['data'];
		// Get stock quantity
		$stock_qty = $product->get_stock_quantity();
		// get cart item quantity
		$item_qty = $cart_item['quantity'];


		if ( ( $stock_qty - $item_qty ) < - 200 ) {

			WC()->cart->remove_cart_item( $cart_item_key );
			$removed_products[] = $product;
		}
	}

	if ( ! empty( $removed_products ) ) {
		//wc_clear_notices(); // remove any WC notice about sorry about out of stock products to be removed from cart.

		foreach ( $removed_products as $idx => $product ) {
			$product_name         = $product->get_title();
			$removed_products_msg = sprintf( __( "The product '%s' was removed from your cart because it is out of stock.", 'woocommerce' ), $product_name );

			wc_add_notice( $removed_products_msg, 'error' );
		}
	}
}