In Tsakonian (a modern descendant of Doric Greek), the letter combination ΣΧ is pronounced
[ʃ] (the first sound in English "ship").
However, it seems clear that this wasn't the ancient pronunciation, since this combination wasn't used to transcribe Hebrew
[ʃ] in the Septuagint.
When did this pronunciation start? Or is it a purely orthographic device, not applying to words that naturally have sigma and chi together like σχίσμα (schisma)?