Subpages of hidden pages with extendToSubpages activated cannot be accessed even with backend login
Tested with: Vanilla TYPO3 9.5.3 and Vanilla TYPO3 8.7.17 and TYPO3 8.7.21
Hidden pages can generally only be accessed with backend login (or the configuration of the admin panel will define the behaviour if it is activated).
If there is a page A with a subpage page B and page A is hidden then this does not affect page B. Page B can still be accessed without backend login.
But if page A also has extendToSubpages enabled then page B cannot be accessed even with backend login (404-handling will define what happens). The desired behaviour would be that it is possible to deactivate pages recursively with one action. Currently this is not possible.