Answer
There's no native toggle to exclude a specific custom field from Copy Order's behavior — the practical workaround is a Custom Event triggered on save of the newly copied order's lines, which explicitly clears/resets the fields you don't want carried over. Two caveats: if the custom field is read-only, it needs to be made editable first, since an event can't clear a field type that isn't editable; and if the same custom field is shared across other windows/LUs, clearing it this way could have side effects there too, so check for that before relying on this approach. Since the insertion behavior itself can't be overridden through configuration, the event action typically needs to run as a background job, which introduces a brief delay before the copied field values actually get cleared on the new order.