I apologize for the inconvenience this change has caused.
As a workaround, you could add some code to manually deconstruct the receipt and decode the developerPayload. Then you could reconstruct the receipt in the format your server accepts. This isn’t ideal, but hopefully it will get you to a place where you are able to continue using Unity IAP.
If you have a project that has upgraded to v1.19 and you need to return it to the previous behavior, you can find previous versions available for download here:
I will pass your feedback along to the IAP team.