Individual 403 and 404 handling in TYPO3 9 and 10. Show a 404 page but keep wrong URL. In case of a missing frontend login, redirect to login page and redirect back after login.

config.yaml

base: 'https://%env(HOST)%/'
baseVariants: {  }
errorHandling:
  -
    errorCode: '403'
    errorHandler: PHP
    errorPhpClassFQCN: Vendor\Sitepackage\PageHandler\NotAuthorized
  -
    errorCode: '404'
    errorHandler: PHP
    errorPhpClassFQCN: Vendor\Sitepackage\PageHandler\NotFound

NotAuthorized.php

<?php
declare(strict_types=1);
namespace Vendor\Sitepackage\PageHandler;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use TYPO3\CMS\Core\Error\PageErrorHandler\PageErrorHandlerInterface;
use TYPO3\CMS\Core\Http\RedirectResponse;
use TYPO3\CMS\Core\Http\Uri;
use TYPO3\CMS\Core\Site\Entity\Site;
use TYPO3\CMS\Core\Site\Entity\SiteLanguage;
use TYPO3\CMS\Core\Site\SiteFinder;
use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * Class NotAuthorized
 * to show a login page (in fitting language) and redirect to page after login
 * @noinspection PhpUnused
 */
class NotAuthorized implements PageErrorHandlerInterface
{
    /**
     * @var int
     */
    protected $pageLoginUid = 167;

    /**
     * @var ServerRequestInterface
     */
    protected $request = null;

    /**
     * @param ServerRequestInterface $request
     * @param string $message
     * @param array $reasons
     * @return ResponseInterface
     */
    public function handlePageError(
        ServerRequestInterface $request,
        string $message,
        array $reasons = []
    ): ResponseInterface {
        $this->request = $request;
        return new RedirectResponse($this->getLoginUrl(), 302);
    }

    /**
     * @return string
     */
    protected function getLoginUrl(): string
    {
        /** @var Site $site */
        $siteFinder = GeneralUtility::makeInstance(SiteFinder::class);
        $site = $siteFinder->getSiteByPageId($this->pageLoginUid);
        /** @var Uri $uri */
        $uri = $site->getRouter()->generateUri(
            $this->pageLoginUid,
            [
                '_language' => $this->getLanguageIdentifier(),
                'redirect_url' => $this->request->getUri()->__toString()
            ]
        );
        return $uri->__toString();
    }

    /**
     * @return int
     */
    protected function getLanguageIdentifier(): int
    {
        /** @var SiteLanguage $language */
        $language = $this->request->getAttribute('language');
        return $language->getLanguageId();
    }
}

NotFound.php

<?php
declare(strict_types=1);
namespace Vendor\Sitepackage\PageHandler;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use TYPO3\CMS\Core\Error\PageErrorHandler\PageErrorHandlerInterface;
use TYPO3\CMS\Core\Http\HtmlResponse;
use TYPO3\CMS\Core\Http\RequestFactory;
use TYPO3\CMS\Core\Http\Uri;
use TYPO3\CMS\Core\Site\Entity\Site;
use TYPO3\CMS\Core\Site\Entity\SiteLanguage;
use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * Class NotFound
 * to show a 404 page (in fitting language)
 * @noinspection PhpUnused
 */
class NotFound implements PageErrorHandlerInterface
{
    /**
     * @var int
     */
    protected $pageNotFoundUid = 133525;

    /**
     * @var ServerRequestInterface
     */
    protected $request = null;

    /**
     * @param ServerRequestInterface $request
     * @param string $message
     * @param array $reasons
     * @return ResponseInterface
     * @throws RequestFailedException
     */
    public function handlePageError(
        ServerRequestInterface $request,
        string $message,
        array $reasons = []
    ): ResponseInterface {
        $this->request = $request;
        return new HtmlResponse($this->getPageNotFoundContent(), 404);
    }

    /**
     * @return string
     * @throws RequestFailedException
     */
    protected function getPageNotFoundContent(): string
    {
        $url = $this->getPageNotFoundUrl();
        $requestFactory = GeneralUtility::makeInstance(RequestFactory::class);
        /** @var ResponseInterface $response */
        $response = $requestFactory->request($url, 'GET');
        if ($response->getStatusCode() === 200) {
            if (strpos($response->getHeaderLine('Content-Type'), 'text/html') === 0) {
                return $response->getBody()->getContents();
            }
        }
        throw new \LogicException('Could not read content of ' . $url, 1594134183);
    }

    /**
     * @return string
     */
    protected function getPageNotFoundUrl(): string
    {
        /** @var Site $site */
        $site = $this->request->getAttribute('site');
        /** @var Uri $uri */
        $uri = $site->getRouter()->generateUri($this->pageNotFoundUid, ['_language' => $this->getLanguageIdentifier()]);
        return $uri->__toString();
    }

    /**
     * @return int
     */
    protected function getLanguageIdentifier(): int
    {
        /** @var SiteLanguage $language */
        $language = $this->request->getAttribute('language');
        return $language->getLanguageId();
    }
}

Thanks for sharing! Not authorized works like a charm. Not found is not needed anymore in TYPO3 10 and later (URL is not changed when using Page Handler).