It happens, feeds go silent. When you're following dozens, or more, you can miss this and it leaves clutter. I would like to see an option that allows the ability to set a number of days before a feed is marked as dead and moved to a separate folder, or tagged as dead until a new unread post appears.
I've noticed a bunch of these. One thing I've found in at least a few cases is that taking the old site and/or feed URL and changing http: to https: will trigger the expected redirect, which is not unsurprising given how heavily Google is promoting HTTPS for search ranking. Perhaps it would be useful to have a dead feed report which gave the option of trying that.
Sorry I didn't update this thread but this launched as part of the Organizer. You can see all of your dead feeds easily. And if you use the All Sites views, you can easily scroll through and find the misbehaving feeds. But the Organizer will allow sorting feeds by last story date, including per-folder or flat across all feeds.