Magento 2.3.0: Implement below code to skip the CSRF check on your custom route called outside Magento environment. This implementation does not break core frontend/adminhtml routes, Magento 2.3/2.2/2.1 web stores.

CsrfValidatorSkip.php

<?php
namespace Module\Vendor\Plugin;

class CsrfValidatorSkip
{
    /**
     * @param \Magento\Framework\App\Request\CsrfValidator $subject
     * @param \Closure $proceed
     * @param \Magento\Framework\App\RequestInterface $request
     * @param \Magento\Framework\App\ActionInterface $action
     */
    public function aroundValidate(
        $subject,
        \Closure $proceed,
        $request,
        $action
    ) {
        if ($request->getModuleName() == 'Your_Module_frontName_Here') {
            return; // Skip CSRF check
        }
        $proceed($request, $action); // Proceed Magento 2 core functionalities
    }
}

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\Request\CsrfValidator">
        <plugin name="csrf_validator_skip" type="Module\Vendor\Plugin\CsrfValidatorSkip" />
    </type>
</config>