There was a plugin we still use for our vBulletin - Self Destruct:
With that plugin it is possible, to mark single forums and sub-forums for self-destruction and to set a time peroid (eg. 5 days). All new threads started in these forums will be marked for self-destruction. Users kann still post replies till the very last moment. At the end of the time period the thread will be completely moved to a special closed and/or hidden sub-forum, the trash bin of my forums which is manually emptied after the high season.
A friend of mine even improved the plugin a little. Additionally the moderators and administrators can mark any thread in any forums for self-destruction, if they recognize that there is only small-talk and chatting in a thread. These threads will be moved within a shorter time period (normally 2 days) to the trash forum too, even if they were not started in a sub-forum which was set for self-destruction.
Whenever a thread is marked for self-destruction, the users see a small clock icon and the time period, when the trhread we be deleted (moved).
Is there any chance, that anybody is writing such an add-on for xenForo, please? What would be needed to do so or to motivate one of you cracks?!