What sort of crime was punished by Scaphism?

You’ve linked to (and read) the English language Wikipedia article in the Question Details. From the English and German Wikipedia articles, we actually don’t know anything else about scaphism: it was described once in Plutarch, and then recapitulated in Eunapius and Zonaras, Byzantine sources. We don’t even know if it was something the Persians actually did, or something Ctesias (the source Plutarch likely cited) made up as a tall tale.

We know that the incident Plutarch cited was the murder of Mithridates (soldier) for killing Artaxerxes II of Persia’s brother Cyrus the Younger (even though Mithridates killed Cyrus accidentally, while the Cyrus was fighting to depose Artaxerxes). Given how spectacular the execution was, it’s reasonable to assume that, if real, it was reserved for the crime the king found most offensive: regicide (or at least, murder of a member of the royal family). Regicide does tend to attract spectacular executions, as occurred in France—and indeed, post mortem punishment, as occurred with Cromwell.