Dispatch an event when a payment is refunded

True that there isn't one, you could detect this via code by implementing an event subscriber reacting to the payment update event.
Commerce would likely need to do the same if it were to provide one as the refundPayment is handled by the payment gateway plugin.

Your event subscribe could check that $payment->original->refunded_amount is not equal to $payment->refunded_amount. You could use the field method for that.

So something like:
if (!$payment->original->get('refunded_amount')->equals($payment->get('refunded_amount')