I checked a customer in Shopify that does not have an email, nor do they have any opt-in set. See screenshot below:
It appears customer with phone and address have been synced. This was the during the initial sync when using Re-import.
Can someone please confirm what fields in Shopify trigger a customer to import during Re-import? I was only expecting customers with emails but I do see value in getting these other customers, since an email may be captured in the future for them and getting their history is useful.