Matt and Trey never censored South Park. The networks airing their show do the censoring as required by their broadcast licenses. A broadcast license is relative to the country that issues it. An Americian station's license is not govern by licenses required in Canada, even if their air waves cross the border.
So you can watch an episode in Canada from the US and see content/language that you might not see on a Canadian channel, and vice versa.
You can see these shows aired without censorship when the broadcast uses data encryption to deliver the content. This might include networks like The Movie Network (TMN) and Comedy Central's subscription channels.
Channels/air waves that employ encryption are not governed by public airwaves standards of their licenses, because the delivery is to targeted subscribers who pay to decrypt the signal.
So on those channels you can see porn, hear all language, etc, etc.
Comedy Central is a subscription channel delivered privately to satellite/cable viewers who pay for the subscription. They likely switched over to encryption after digital TV became standard.