Why does almost every example I can find (including this question from about a year ago) say that a 404 header should be HTTP/1.0 404 Not Found when we've really been using HTTP 1.1 for over a decade? Is there any reason not to send HTTP/1.1 404 Not Found instead?
(Not that it matters all that much... I'm mostly just curious.)
The usage of HTTP version can be based on the following factors:
Modern browsers can support both 1.0 and 1.1 well, and both the client and server will settle for the highest version both can support together. The key differences between the 2 protocol can be found: http://www8.org/w8-papers/5c-protocols/key/key.html
However there's no key differences in the usage of 404 Not Found. However do be consistent for your whole website. i.e. if you use HTTP/1.1, you use it throughout your website.