Can I change the message shoppers see when their pincode is not serviceable?

Yes. Go to Plugins → Delivery Restrictions and stay on the Settings tab.

• Serviceable message is shown when the pincode can be delivered to.
• Not-serviceable message is shown when it cannot, including when one of the products in the cart is not allowed for that pincode.

Type your own wording into each box and click Save settings.

The messages appear wherever the pincode checker is switched on: Show on product page, Show on cart and Show on checkout. If you also tick Show delivery estimate and fill in Default delivery days, serviceable pincodes show a delivery estimate as well.