Magento 2 Custom No Route Handler redirect to default 404 Not Found Page

Question

I am trying to redirect url to my custom no route found page but it always redirect to default 404 page,

I have made below changes in my module

Step-1 Create below file in Controller Folder CustomNoRouteHandler.php

<?php

namespace SimplifiedMagento\RequestFlow\Controller\Page;

class CustonNoRouteHandler implements \Magento\Framework\App\Router\NoRouteHandlerInterface
{
    public function process(\Magento\Framework\App\RequestInterface $request)
    {
        $request->setRouteName('noroutefound')->setControllerName('page')->setActionName('customnoroute');

        return true;
    }
}

Step-2 My redirect page which should be display while no route found,

<?php

namespace SimplifiedMagento\RequestFlow\Controller\Page;

use Magento\Framework\App\Action\Action;
use Magento\Framework\App\ResponseInterface;

class CustomNoRoute extends Action
{
    public function execute()
    {
        echo "this is our custom 404 page";
    }
}

Step-3 In etc/frontend/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">
    <type name="Magento\Framework\App\Router\NoRouterHandlerList">
        <arguments>
            <argument name="handlerClassList" xsi:type="array">
                <item name="handlerinfo" xsi:type="array">
                    <item name="class" xsi:type="string">SimplifiedMagento\RequestFlow\Controller\Page\CustonNoRouteHandler</item>
                    <item name="sortOrder" xsi:type="string">50</item>
                </item>
            </argument>
        </arguments>
    </type>
</config>

Step-4 In etc/frontend/routes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
    <router id="standard">
        <route id="noroutefound" frontName="noroutefound">
            <module name="SimplifiedMagento_RequestFlow" />
        </route>
    </router>
</config>

Answer 1

I have done the same custom 404 page for one of my extension.

I see a difference of code in Route php file process method.

Your ones is

$request->setRouteName('noroutefound')->setControllerName('page')->setActionName('customnoroute');

While i found in my code is

$request->setModuleName('noroutefound')->setControllerName('page')->setActionName('customnoroute');

Also put di.xml inside etc/di.xml insetad of etc/frontend/di.xml

Try this change once if that work for you.