A module to add products to Magento 2's topmenu

etc-events.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * @author: Daan van den Bergh
 * @url: https://daan.dev
 * @package: Dan0sz/TopmenuProducts
 * @copyright: (c) 2019 Daan van den Bergh
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="page_block_html_topmenu_gethtml_before">
        <observer name="dan0sz_topmenu_products_gethtml_before" instance="Dan0sz\TopmenuProducts\Observer\TopmenuGetHtmlBefore" />
    </event>
</config>

etc-module.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * @author: Daan van den Bergh
 * @url: https://daan.dev
 * @package: Dan0sz/TopmenuProducts
 * @copyright: (c) 2019 Daan van den Bergh
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Dan0sz_TopmenuProducts" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Catalog" />
        </sequence>
    </module>
</config>

Observer-TopmenuGetHtmlBefore.php

<?php
/**
 * @author   : Daan van den Bergh
 * @url      : https://daan.dev
 * @package  : Dan0sz/TopmenuProducts
 * @copyright: (c) 2019 Daan van den Bergh
 */

namespace Dan0sz\TopmenuProducts\Observer;

use Magento\Catalog\Model\Product;
use Magento\Catalog\Model\ResourceModel\Product\Collection as ProductCollection;
use Magento\Catalog\Model\ResourceModel\Product\CollectionFactory as ProductCollectionFactory;
use Magento\Framework\App\Request\Http as Request;
use Magento\Framework\Data\Tree\Node;
use Magento\Framework\Data\Tree\NodeFactory;
use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class TopmenuGetHtmlBefore implements ObserverInterface
{
    /** @var ProductCollection $productCollection */
    private $productCollection;

    /** @var Node $node */
    private $node;

    /** @var Request $request */
    private $request;

    /**
     * TopmenuGetHtmlBefore constructor.
     *
     * @param ProductCollectionFactory $productCollection
     * @param NodeFactory              $node
     */
    public function __construct(
        ProductCollectionFactory $productCollection,
        NodeFactory $node
    ) {
        $this->productCollection = $productCollection;
        $this->node              = $node;
    }

    /**
     * @param Observer $observer
     *
     * @return $this|void
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function execute(Observer $observer)
    {
        /** @var Node $menu */
        $menu = $observer->getData('menu');
        /** @var Request request */
        $this->request = $observer->getData('request');
        $products      = $this->getTopmenuProducts();
        $this->addProductsToMenu($products, $menu);

        return $this;
    }

    /**
     * @return ProductCollection
     */
    private function getTopmenuProducts()
    {
        /** @var ProductCollection $productCollection */
        $productCollection = $this->productCollection->create();
        $productCollection->addAttributeToSelect(
            [
                'status',
                'entity_id',
                'name',
                'url_key'
            ]
        );
        // This is where you add the entity_id's of the products you want to add to your menu.
        $productCollection
            ->addAttributeToFilter('entity_id', ['in' => [1, 2, 3, 4]]);
        // And perhaps another attribute by which you want to sort?
        $productCollection->addAttributeToSort('entity_id');

        return $productCollection;
    }

    /**
     * @param ProductCollection $products
     * @param Node              $menu
     *
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    private function addProductsToMenu(ProductCollection $products, Node $menu)
    {
        foreach ($products as $product) {
            $node = $this->node->create(
                [
                    'data'    => $this->getProductAsArray($product),
                    'idField' => 'id',
                    'tree'    => $menu->getTree()
                ]
            );
            $menu->addChild($node);
        }
    }

    /**
     * @param Product $product
     *
     * @return array
     */
    private function getProductAsArray(Product $product)
    {
        return [
            'name'             => $product->getName(),
            'id'               => 'product-node-' . $product->getId(),
            'url'              => $product->getProductUrl(),
            'has_active'       => false,
            'is_active'        => $this->checkIsActive($product),
            'is_category'      => false,
            'is_parent_active' => true
        ];
    }

    /**
     * @param Product $product
     *
     * @return bool
     */
    private function checkIsActive(Product $product)
    {
        return $product->getId() == $this->request->getParam('id');
    }
}

registration.php

<?php
/**
 * @author: Daan van den Bergh
 * @url: https://daan.dev
 * @package: Dan0sz/TopmenuProducts
 * @copyright: (c) 2019 Daan van den Bergh
 */

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Dan0sz_TopmenuProducts',
    __DIR__
);