This news is from a couple of weeks ago, but is still worth mentioning. A representative from Google has stated that they now view a 410 HTTP status code as “more permanent” than a 404 code.
The difference between the two is that a 404 status code returns a message of “Not Found” from the web server, whilst the 410 status code returns a message of “Gone”. This message is received if the URL no longer exists, the URL is incorrect, or possibly if content has become temporarily unavailable. Historically Google have handled these responses as meaning the same, and the change to differentiate them is a recent one.
I can imagine the reason for this is redundant content still being linked to, returning a 404 response, and therefore remaining accessible to search engine crawlers for them to attempt to index the content. A differentiation with the 410 code may help the search engine to identify content which should not be crawled and allow them to drop it out of the index altogether.
Of course, if you have content within your site returning 404 or 410 and they have strong links pointing to them resulting in them still being crawled, you will want to 301 redirect these back into your site to ensure you maintain that link equity.
The quote from Google’s John Mu in full:
I followed up on the 404 vs 410 thing with the team here. As mentioned by some others here & elsewhere, we have generally been treating them the same in the past.
However, after looking at how webmasters use them in practice we are now treating the 410 HTTP result code as a bit “more permanent” than a 404. So if you’re absolutely sure that a page no longer exists and will never exist again, using a 410 would likely be a good thing. I don’t think it’s worth rewriting a server to change from 404 to 410, but if you’re looking at that part of your code anyway, you might as well choose the “permanent” result code if you can be absolutely sure that the URL will not be used again. If you can’t be sure of that (for whatever reason), then I would recommend sticking to the 404 HTTP result code.
In the worst case, the 410 will be treated the same as a 404; in the best case it’ll be a bit quicker & stickier :-).