What happens to the application materials sent/submitted with graduate school applications in US after admissions decisions are made? After the admission decisions are announced, accepted students made their decision, and the list of next year's graduate class is determined, what do graduate schools and respective departments in US universities typically do with the application materials (electronic or paper), including recommendation letters, transcripts and score reports, that they received from each individual applicant, in particular, rejected applications?

Do they keep them, in their entirety, for future reference or statistics? For how long, typically? Or, do they keep a summary of each application and the reasons for rejection and discard the rest?
At our school, we never keep written reasons for acceptance or rejection. These are made in a final faculty meeting with voice votes of yays and nays for each candidate that made it to the semi-finalist list.  Only the finalist list is recorded.

As to materials, they were shredded sometime after admissions season is over. It's a moot point now that everything is electronic.