The ‘Last-Modified’ response header contains information regarding the date and time that a webpage was last modified. This information is sent by the origin server (the web server that holds the original webpage) when web users access a page.
The Last-Modified response header can be sent by the server to provide a browser with information about when the page was last updated. With this information, the browser can then decide whether the content must be requested again or whether this content still exists in the cache from a previous visit to the site.
The data from the Last-Modified Response header is frequently used by the If-Modified-Since header.
The Last-Modified response header is not always implemented.On WordPress is depends on the way the theme is coded.
Further information on this HTTP response header can be found on the ‘Last-Modified’ Mozilla Developer page.