I saw the dismissals of parts of a couple of academic libraries and they didn't scan the dismissed books. It's just two examples, but I suspect it's the commonest case. The effort to scan lots of books in a short time is huge and the personnel is usually limited (dismissals frequently happen in bulk during reorganisations or restructures).

On the other hand, there are libraries, like the _Biblioteque Nationale de France_, which offer the service of photocopying or scanning rare books at a price that includes the payment of the copyright fees (side note: I actually used that service from another country, and as a memento for those who come here asking how to find rare books, I can say: _learn to use library services!_).