The Wikipedia says that Stack Exchange was:
Launched on September 2009; 9 years ago. (relaunched in January 2011)
Was Stack Exchange network really closed? If so, why?
Stack Exchange, and Stack Overflow has... well evolved over time.
Sadly the original post seems lost, but through the magic of the Internet archives, that's roughly the time that Stack Exchange 2.0 was started and the current system of new site creation. It's around the time Stack Exchange in its current form was mooted.
Amusingly everything old is new again - as far as the post goes (we're still dealing with divergences in site code, you can buy a private SO instance...).
As someone who was there in the period SE was supposedly closed, I can assure you though it was never closed at all, or I was apparently wandering around the internet equivalent of a renovation site.