They aren't, in general. The point about spare space which is unreported is that it's for internal use only - any software accessing the memory through normal methods doesn't know whether the data goes on what was previously spare space, space which has never been used before, or if it's overwriting some data which is marked as deleted.
If the memory unit (drive/chips/whatever) has some specific "wipe" method, it might be that this specifically overwrites spare space, but the only way to check would be to access the data in a non-standard way, since the normal methods would just show the data on the currently readable parts. You'd also have to trust that the method which was provided by the manufacturer actually did what it said it did.
However, if you've used full disk encryption (or equivalent - full device encryption, perhaps, with a phone), it all becomes academic. If the keys are destroyed, any data is practically unreadable, whether you can pull it off spare space or not. That's why the general advice for SSD/flash memory use is to encrypt before writing anything important to disk - it means that even if there is some data you can no longer overwrite reliably, it still can't be used.