Change effective on 31 October 2018
Rails applications using the asset pipeline utilize a “fragment cache” located at
tmp/cache/assets/sprockets/ to improve asset generation time. If your app is not dynamically generating assets in runtime due to setting
config.assets.compile = false, then these files are not needed at runtime and can be removed.
The goal of removing this directory is to have smaller slug sizes and faster boot times. More information can be found on the asset pipeline caching docs.