"In a year since Facebook’s blog about Cache-Control Immutable, we can see that it’s usage has spread beyond Facebook and is being used by a handful of large 3rd parties and ~4150 additional domains. Across all of the pages in the HTTP Archive, 2% of requests and 30% of sites appear to include at least 1 immutable response. Additionally, most of the sites that are using it have the directive set on assets that have a long freshness lifetime."