behat.yaml

default:
    extensions:
        YourVendor\YourPackage\Behat\Extension\ClassLoaderExtension: ~
        #
        # some other extensions and setup
        #

ClassLoader.php

<?php
// YourVendor/YourPackage/Behat/Legacy/ClassLoader.php
declare(strict_types=1);

namespace YourVendor\YourPackage\Behat\Legacy;

use Symfony\Component\ClassLoader\ClassLoader as SymfonyClassLoader;

try {
    require_once 'path/to/vendor/symfony/class-loader/ClassLoader.php';
} catch (\Throwable $exception) {
    //
}

class ClassLoader extends SymfonyClassLoader
{
}

ClassLoaderExtension.php

<?php
declare(strict_types=1);

namespace YourVendor\YourPackage\Behat\Extension;

use Behat\Testwork\ServiceContainer\Extension;
use Behat\Testwork\ServiceContainer\ExtensionManager;
use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Definition;
use YourVendor\YourPackage\Behat\Legacy\ClassLoader;

final class ClassLoaderExtension implements Extension
{
    public function process(ContainerBuilder $container)
    {
    }

    public function getConfigKey()
    {
        return 'behat_workaround';
    }

    public function initialize(ExtensionManager $extensionManager)
    {
    }

    public function configure(ArrayNodeDefinition $builder)
    {
    }

    public function load(ContainerBuilder $container, array $config)
    {
        // Reset 'class_loader' definition to the class we created earlier.
        $container->setDefinition('class_loader', new Definition(ClassLoader::class));
    }
}