Technically, this doesn't fully qualify as a closure, but the story of NPR (Not Programming Related) is pretty much like it. Some time after graduation site was renamed, its scope was heavily redefined1 and lots of old questions were removed:
NPR was supposed to be a site where questions that were too subjective / broad for Stack Overflow would find a new home... It didn't take long for everyone to realize that the site was not working, and most people just didn't bother with it...
First, Joel warned us that the site was "degrading into fairly stupid water-cooler nonsense" and then Jeff stepped in, and enforced the infamous subjective guidelines. It took more than a few months for people to realize that NPR's (by then already renamed to Programmers) scope had changed drastically...
Then came the clean-ups... we cleaned up all the content that no longer fit the site's scope. After a few months of intense Meta drama, we deleted about 2K questions. And then some...
The site... existed for more than a few months, and it failed. Horribly....
Further reading
1 Technically, site renaming occurred and attempt to redefine scope started prior to graduation but detailed story indicates that the real shift happened much later and took place between Dec '10 and April '12 or even longer.