I have a question about the legal system of the Medo-Persian empire. Is that on-topic here?
Such question is definitely on topic here.
However, most of the regular posters may know little about a system that far back in history. We have had good answers about the laws of the middle ages, and about some aspects of Roman law, if I recall correctly. One might get better answers about the laws of the Medo-Persian empire on History.SE, but there is no harm in trying here.
The appropriate category is legal-history. Answers might come in very sparingly though.
I'm a moderator on History Stack Exchange, where we have a law tag (Currently at 306 questions).
Even though ancient law is on-topic both at History and here, there is a difference in focus. History focuses on answering questions using historical methods, such as consulting history textbooks and journals, first-hand accounts, official histories, manuscripts, epigraphy, etc. If you ask a question there on the state of Roman law, it is far more likely that an answer will be given from the writings of Roman historians such as Tacitus or Suetonius than from an analysis of Roman statutes, cases, and legal principles.
A while back, I asked the question What was the mens rea of being a common scold? . I asked it here rather than on History because I was specifically interested in a legal analysis of mens rea rather than information on the historical context of the offense or how it was used in practice to exert social control against undesirables and dissidents.