If I remember correctly, the last episode with Kenny was when he was dying from a disease that only stem cell research could cure. Cartman lobbied Congress to allow stem cell research, but, being Cartman, he did it not to save Kenny but to "clone" his own Stuckey's restaurant. Kenny died, and the kids all recognized it as a "real" death (as opposed to Kenny's recurring death-of-the-week). I was surprised by that, and even more surprised when Kenny never came back on subsequent episodes. I wouldn't put it past Trey Parker and Matt Stone for this to be a long-term hoax, though.