You can easily listen to webhooks for subscription cancellations, and you can also poll the API for subscription information - ideally both to guarantee consistency (watch+list pattern). Any non-trivial integration would have this. Don't blame end users for your tech #fails.

Even as I point this out I don't seem to understand your attitude, unless it's on behalf some lame shared cpanel hosting providers using some crappy billing software like whmcs - those providers lame everything on the customer.