Okay, so from what I have discovered, seems that Instant Feedback
service from Apple is working just fine and is able to detect uninstall pretty fast. There was a small trick in how to test these things properly with sandbox service.
If you make an app which you are testing locally on your device and your app is the only one installed on your phone from that provisioning profile (vendor), stuff which I faced as an issue in my question will be encountered. For some reason, even if you uninstall the app (which was the only one on the phone installed from your provisioning profile), Apple is always responding with 200.
What needs to be done is following: After you install your app which you are testing, create a new dummy app with another bundle ID, but which also coming from your provisioning profile, enable push notifications in it as well and install it on your device. So now, you have two apps that you made - one which you are developing and testing the Instant Feedback
service with and another one which kinda "zombie" app which just sits in there and does nothing.
After this, uninstall your test app, try to send silent push notification and Apple will respond with status code 410
as expected.
Dummy way, but for some reason that's how it should be while testing. Behaviour in production should be as expected and w/o any need to have a dummy (or any other) app from same vendor installed on iOS device.
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