This domain should be reused, and should redirect to the current year's Winter Bash.
Why did I think this would happen? The use of this domain was announced in the original email to moderators, which read in relevant part:
This event will run from 16 December 2013 to 3 January 2014. Users will be able to see their entire hat collection on http://winterba.sh. That site will also have a landing page, explaining the rules and other details of the event.