The idea that if a technology exists in the real world questions about it are automatically off-topic is just bizarre. As William Gibson has said, "The future is already here — it's just not very evenly distributed." Of course the idea that science fiction is about futuristic technology is equally problematic...
Questions about technology may or may not be on-topic, but I don't think that will depend on whether they exist in the real world. Remember that "setting explanations" as well as the "Historical or societal context of a work" are specifically on-topic. Questions about technology should be judged on the normal criteria: is it focused and clear.
I think we can distinguish between questions asking about the references to a technology in works of sci-fi/fantasty and references to a technology in any genre, or even in the real world. But asking about poison lipstick or rockets or mobile phones in works of sci-fi, as long as the question is focused and clear, should be on-topic regardless of their real-world existence.