TYPO3 Core

Additional note: My group field is in the TCA of table pages, if this was not clear.

Code to test (TCA/Overrides/pages.php):

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

call_user_func(function () {
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTCAcolumns('pages',
        [
            'group_db_pages' => [
                'label' => 'group_db_pages allowed=pages',
                'description' => 'pages on current page',
                'config' => [
                    'type' => 'group',
                    'internal_type' => 'db',
                    'allowed' => 'pages',
                    'size' => 1,
                    'suggestOptions' => [
                        'pages' => [
                            'addWhere' => 'AND pid = ###CURRENT_PID###'
                        ]
                    ]
                ]
            ]
        ]
    );
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addToAllTCAtypes('pages', 'group_db_pages');
});

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/63587