ylsine

Do you have a blog that really only want ready by humans? Do think that 
<a href="http://google.com/" rel="nofollow ugc">robots</a> should never be able to read your blog, even if they ignore your robots.txt? Is privacy your number one concern? Do your friends think of you as “that total privacy nut…you know too much so.”? Then this plugin is for you. The 404er will make every public page of your blog return a <a href="http://en.wikipedia.org/wiki/HTTP_404" rel="nofollow ugc">404</a> instead of the normal <a href="http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#2xx_Success" rel="nofollow ugc">200</a>. Yes, you could use the normal <a href="https://codex.wordpress.org/Settings_Privacy_SubPanel" rel="nofollow ugc">privacy subpanel</a>, but that would be sooo pedestrian. Step up to the real (excessive, <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5" rel="nofollow ugc">RFC abbusing</a>) 404!
Note: If you did not detect any humor in the above text or you do not know what a 404 is, then, in all caps, DO NOT INSTALL THIS PLUGIN!

The 404er

This plugin will send a 404 header for every public page on your blog.

zubovd

This plugin adds the ‘Last-Modified’ header to each post and returns the ‘304 Not Modified’ header to the client without a body in the response if he sent the ‘If-Modified-Since’ header and the post has not changed since that date.

Last-Modified and If-Modified-Since Headers

Add Last-Modified anв support If-Modified-Since Headers

The 404er vs Last-Modified and If-Modified-Since Headers