I have successfully registered an app uninstalled webhook from api call on app installation. (The json return success,). I see the webhook successfully being registered, however when I delete the app from test Shopify store, I do not see that webhook being triggered.
the only case that you register webhook URL that was not up or not working. this the only case webhook not fire. shopify send webhook after some retry 10 then then time. if failed. you need to return the success response as per the shopify guide
more detail you can see thie guide https://shopify.dev/docs/apps/webhooks/configuration/https#step-4-receive-the-webhook