vB stores read/unread-information as a timeline per thread per user, i.e. it safes the time the user last looked at the thread. One would have to get that time, then count all posts in the post table that have been posted to a said thread after that time. Since this would have to be done with every load of showthread, it has the potential to add a lot of additional strain on the database, especially the post table. It probably is possible, but I would definitely advise against it.
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