Our design is for our backend to make the paymentSession call and return the response to the frontend. However, we are doing an exact field mapping while reading the response, which meant the backend errored out validation when it couldn't find domainName in the response. In addition, the "new" fields (like initiativeContext) being returned weren't mapped and hence not sent back to frontend, meaning signature validation later on would have failed as well.

We can see the response format has now reverted back to the "old" format. Until that, ApplePay in our preprod environments was fully down. Would this change happen again? If so, is there a possibility of an announcement or notification so we can be better prepared?

Understood, thank you! This has been our behaviour for years, but we will change our design as suggested. This change will require some time tho, so I hope the response format doesn't change again soon.