flatpress/fp-includes/smarty-4.3.1/docs/programmers/api-variables/variable-cache-modified-check.md

$cache_modified_check

If set to TRUE, Smarty will respect the If-Modified-Since header sent from the client. If the cached file timestamp has not changed since the last visit, then a '304: Not Modified' header will be sent instead of the content. This works only on cached content without {insert} tags.

See also $caching, $cache_lifetime, and the caching section.