ext_localconf.php

<?php
defined('TYPO3_MODE') || die('Access denied');

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\TYPO3\CMS\Core\DataHandling\DataHandler::class]['className'] =
    \NamelessCoder\AsyncReferenceIndexing\DataHandling\DataHandler::class;

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\TYPO3\CMS\Core\Database\ReferenceIndex::class]['className'] =
    \NamelessCoder\AsyncReferenceIndexing\Database\ReferenceIndex::class;

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['extbase']['commandControllers'][] =
    \NamelessCoder\AsyncReferenceIndexing\Command\AsyncReferenceIndexCommandController::class;

$_EXTCONF = unserialize($_EXTCONF);
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY] = $_EXTCONF;

// Register signal
$dispatcher = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Extbase\SignalSlot\Dispatcher::class);
$dispatcher->connect(
    \TYPO3\CMS\Core\Database\ReferenceIndex::class,
    'shouldExcludeTableFromReferenceIndex',
    \NamelessCoder\AsyncReferenceIndexing\Slot\ReferenceIndexSlot::class,
    'shouldExcludeTableFromReferenceIndex'
);