I just got a notification via the app for a new comment (on Stack Overflow) that was added to a question that was put on hold and subsequently deleted by the OP.
When I clicked the notification, the app opened with an appified 404:
Page Not Found
We couldn't find the page you requested
Open in Chrome
If you don't want to open deleted questions in the app (which I can do in browser), then show something better than a 404 message, such as
Sorry, that post has been deleted, and this app doesn't support viewing deleted posts