I recently posted a code-golf answer in Scheme-1. However, the Scheme-1 language cannot be implemented (as it would require the implementation to solve the halting problem).
I marked it as non-competing because there is no implementation. However, @ais523 commented:
I'm not sure that you can submit an answer in a language that's theoretically impossible to implement (assuming the Church-Turing hypothesis), because we normally go by the implementation to define how the language works, and that doesn't work in this case. Non-competing status is normally reserved for languages where there's a viable implementation, but it's newer than the challenge.
What is our policy on uncomputable submissions?