Shared hosting servers return the HTTP header by default
Last-Modified both for static (images, style sheets, JS scripts) and dynamic content (dynamic pages, PHP scripts). If you are faced with a situation in which the server does not return this header, you should check if the "SSI" in site settings:
If the option "SSI"Is enabled, the server will not return a header for dynamic content. For static content, the title will be returned anyway. Technology work SSI provided thanks to Apache web server modules, which we use as a backend server on the hosting. IN official documentation reported that when using SSI, the header
Last-Modified not returned because it is difficult to calculate the last modified time for dynamic content.