Just over a year ago, I permanently redirected all my feeds their Atom 1.0 equivalents. Several months later, I quietly converted all those redirects to 410 Gone.
Checking back to see how effective this has been, here is a list of user agents that are still attempting to fetch those missing feeds: by name and by full User-Agent. The numbers down the left are the number of requests over the past week.
The most interesting line in that output:
Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; 450 subscribers; feed-id=16685407958911290868)