Over the course of 2013 & up until September 2014 we migrated roughly a dozen domains to a new platform. In most cases the domain names remained the same, we simply pointed the domain to a new IP address where the new appservers were located. Since the underlying architecture was significantly different we put in place upwards of 500 redirects (all 301's) so that users & bots would be directed properly to the new pages, and over time all the old links have been replaced in Google, Bing, etc. with the new links.
I recently did an analysis of our web server logs and found that in the past 30 days these redirects have been hit over 211,000 times, with over 208,000 of them identifying themselves as various bots from their User-Agents. One particular bot seems to only ever hit URL's that result in a 301 response and never proceeds further. I can't find an entry in our logs for this particular UA that results in a 2xx, 4xx, or 5xx response.
Given that we've had these redirects in place for anywhere from 9 months to 2 years and that the vast majority of traffic hitting them are bots (in many cases they are exclusively being hit by bots), does it make sense to change these from say a 301 to a 410 (Gone) to inform these bots that the URL's are gone for good? I'd like to eventually do away with all these redirects if I can since they just add to the complexity of our configurations.