“Swift for TensorFlow was an experiment in the next-generation platform for machine learning, incorporating the latest research across machine learning, compilers, differentiable programming, systems design, and beyond.”
Google hired Chris Lattner, announced Swift as the future of machine learning and built it into a language fork. Lattner left in 2020. The project got archived months later. Researchers stuck with Python the whole time. Calling it an experiment after the fact is how Google says nobody used it.