Deb died and Dexter drove his sweet-ass boat into a hurricane, faked his death and went somewhere to be a logger. He has a beard now and may have a tough time finding folks in the Pacific Northwest worthy of the delicate work of his dark passenger.
I suspect the actual staff left at the end of season three. A janitor who'd been keeping up jotted down and left behind some ideas he come up with all by his own self. A bit later some trouble making teens broke into the studio and produced subsequent seasons 4-8 as a joke.