TYPO3 Core

With the introduction of getPagesOverlay the page array is looped to pass each page to the getPageOverlay_preProcess hook:

foreach ($pagesInput as $origPage) {
                foreach ($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_page.php']['getPageOverlay'] as $classRef) {
                    $hookObject = GeneralUtility::getUserObj($classRef);
                    if (!$hookObject instanceof PageRepositoryGetPageOverlayHookInterface) {
                        throw new \UnexpectedValueException('$hookObject must implement interface ' . PageRepositoryGetPageOverlayHookInterface::class, 1269878881);
                    }
                    $hookObject->getPageOverlay_preProcess($origPage, $lUid, $this);
                }
            }

However, as the foreach loop doesn't use & before $origPage modified values in getPageOverlay_preProcess will never make it back to the original page array.