If you're going to count all possible side channels as output mechanisms, then I'd imagine that it is indeed impossible to build a computer without an output me

If you're going to count all possible side channels as output mechanisms, then I'd imagine that it is indeed impossible to build a computer without an output mechanism. Unless you limit your scope to say, Earth, and somehow build a computer outside of Earth's light cone. It is thus causally disconnected from Earth and has no output mechanisms that can be read from anyone on Earth.