As a new iOS developer, I was surprised to see that over all the years it has been impossible to be notified if a user had refunded a purchase. I got that from questions like these:
Now as of 2020 it seems Apple has finally introduced a notification for it. But this sounds like it only works with an additional server. I want to make sure I'm not missing something.
I do not have my own server. Can I still somehow detect refunds using only local validation (or a similar concept)?
Yes, you need your own server to accept & respond to the REFUND (webhook) notification(s) coming from Apple's App Store Server.
There is no Apple-provided UI for viewing this data as it is meant to be a programmatic integration with a backend service.
App Store Server Notifications is a server-to-server service as confirmed by official Apple documentation.
The language used in other official documentation, like the one you have linked or this one here, also clearly specifies the requirement for your server.
When the App Store processes a refund, the App Store Server sends a REFUND notification to your server, at the URL you configured. Your server must respond to the post with a 200 response code.
Your server is responsible to parse and interpret all notifications from App Store Server.
When you set up the endpoints on your server to receive notifications, configure your server to send a response.