Get list of all subpages in PHP or alternative to QueryGenerator->getTreeList

You can quickly conjure up an alternative to this function yourself with the QueryBuilder in TYPO3. This can then look something like this:

<?php

declare(strict_types=1);
namespace In2code\Lux\Domain\Repository;

use Doctrine\DBAL\DBALException;
use Doctrine\DBAL\Driver\Exception as ExceptionDbalDriver;
use PDO;
use TYPO3\CMS\Core\Database\ConnectionPool;
use TYPO3\CMS\Core\Database\Query\Restriction\DeletedRestriction;
use TYPO3\CMS\Core\Database\Query\Restriction\HiddenRestriction;
use TYPO3\CMS\Core\Utility\GeneralUtility;

class PageRepository extends AbstractRepository
{
    const TABLE_NAME = 'pages';
    
    /**
     * Successor of TYPO3\CMS\Core\Database\QueryGenerator->getTreeList as it was removed in TYPO3 12
     *
     * @param int $pageIdentifier Start page identifier
     * @param bool $addStart Add start page identifier to list
     * @param bool $addHidden Should records with pages.hidden=1 be added?
     * @return array
     * @throws DBALException
     * @throws ExceptionDbalDriver
     */
    public function getAllSubpageIdentifiers(int $pageIdentifier, bool $addStart = true, bool $addHidden = true): array
    {
        $identifiers = [];
        if ($addStart === true) {
            $identifiers[] = $pageIdentifier;
        }
        foreach ($this->getChildrenPageIdentifiers($pageIdentifier, $addHidden) as $identifier) {
            $identifiers = array_merge($identifiers, $this->getAllSubpageIdentifiers($identifier, true, $addHidden));
        }
        return $identifiers;
    }

    /**
     * @param int $pageIdentifier
     * @param bool $addHidden
     * @return array
     * @throws DBALException
     * @throws ExceptionDbalDriver
     */
    protected function getChildrenPageIdentifiers(int $pageIdentifier, bool $addHidden): array
    {
        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable(self::TABLE_NAME);
        $queryBuilder->getRestrictions()->removeAll();
        $queryBuilder->getRestrictions()->add(GeneralUtility::makeInstance(DeletedRestriction::class));
        $queryBuilder
            ->select('uid', 'uid')
            ->from(self::TABLE_NAME)
            ->where(
                $queryBuilder->expr()->eq(
                    'pid',
                    $queryBuilder->createNamedParameter($pageIdentifier, PDO::PARAM_INT)
                ),
                $queryBuilder->expr()->eq('sys_language_uid', 0)
            );
        if ($addHidden === false) {
            $queryBuilder->getRestrictions()->add(GeneralUtility::makeInstance(HiddenRestriction::class));
        }
        $result = $queryBuilder->execute()->fetchAllKeyValue();
        return array_values($result);
    }
}

This code example was (slightly modified) taken from the LUX extension. Here the start function getAllSubpageIdentifiers() is called recursively until all subpages have been added. At least the start page UID must be passed as a parameter.

Also see the deprecation notice to PHP class QueryGenerator in the TYPO3 documentation.