Magento – Magento2 REST API products list and out of stock

Go to admin > Stores > configuration > Catalog > Inventory > Stock Options

Set Display Out of Stock Products to NO

I've fixed this issue by overriding the below file in our custom module: \Magento\ConfigurableProduct\Helper\Data.php

Below are the complete steps:

1) Add below statement in your module's di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="\Magento\ConfigurableProduct\Helper\Data" type="YOUR_COMPANY\MODULE_NAME\Helper\ConfigurableProduct\Data" />
</config>

2) Create a file Data.php at YOUR_COMPANY\MODULE_NAME\Helper\ConfigurableProduct and put below code in it

    <?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace YOUR_COMPANY\MODULE_NAME\Helper\ConfigurableProduct;

use Magento\Catalog\Model\Product;

/**
 * Class Data
 * Helper class for getting options
 *
 */
class Data extends \Magento\ConfigurableProduct\Helper\Data
{
    /**
     * Catalog Image Helper
     *
     * @var \Magento\Catalog\Helper\Image
     */
    protected $imageHelper;
    protected $_productloader;  
    protected $stockRegistry;  
    protected $_storeManager;

    /**
     * @param \Magento\Catalog\Helper\Image $imageHelper
     */
    public function __construct(
        \Magento\Catalog\Helper\Image $imageHelper,
        \Magento\CatalogInventory\Api\StockRegistryInterface $stockRegistry,
        \Magento\Catalog\Model\ProductFactory $_productloader,
        \Magento\Store\Model\StoreManagerInterface $storeManager        
        )
    {
        $this->imageHelper = $imageHelper;
        $this->stockRegistry = $stockRegistry;
        $this->_productloader = $_productloader;
        $this->_storeManager = $storeManager;
        parent::__construct($imageHelper);
    }

    /**
     * Get Options for Configurable Product Options
     *
     * @param \Magento\Catalog\Model\Product $currentProduct
     * @param array $allowedProducts
     * @return array
     */
    public function getOptions($currentProduct, $allowedProducts)
    {

        $options = [];
        foreach ($allowedProducts as $product) {
            $productId = $product->getId();

            $websiteId = $this->getCurrentWebsiteId();
            if ($websiteId == 2) {
                $product = $this->getLoadProduct($productId);
                $stockitem = $this->stockRegistry->getStockItem($product->getId(), $product->getStore()->getWebsiteId());
                if($stockitem->getQty() <= 0 || $stockitem->getIsInStock() != 1) continue;
            }

            $images = $this->getGalleryImages($product);
            if ($images) {
                foreach ($images as $image) {
                    $options['images'][$productId][] =
                        [
                            'thumb' => $image->getData('small_image_url'),
                            'img' => $image->getData('medium_image_url'),
                            'full' => $image->getData('large_image_url'),
                            'caption' => $image->getLabel(),
                            'position' => $image->getPosition(),
                            'isMain' => $image->getFile() == $product->getImage(),
                        ];
                }
            }
            foreach ($this->getAllowAttributes($currentProduct) as $attribute) {
                $productAttribute = $attribute->getProductAttribute();
                $productAttributeId = $productAttribute->getId();
                $attributeValue = $product->getData($productAttribute->getAttributeCode());

                $options[$productAttributeId][$attributeValue][] = $productId;
                $options['index'][$productId][$productAttributeId] = $attributeValue;
            }
        }
        return $options;
    }

    public function getLoadProduct($id)
    {
        return $this->_productloader->create()->load($id);
    }

    public function getCurrentWebsiteId(){
        return $this->_storeManager->getStore()->getWebsiteId();
    }


    }

I've added condition for website_id as I want to hide out of stock options for 2nd website only. You can remove that condition.

Flush the cache and check any configurable product having out of stock associated products in it.

Thanks!