It kinda looks like the cached data option has been intentionally removed from user settings. These paragraphs from the O dev preview release notes may suggest why:
"Android O gives better guidance and behaviors around cached data. Each app is now given a disk space quota for cached data....
When the system needs to free up disk space, it will start by deleting cached files from apps that are the most over their allocated quota. Thus, if you [the developer of an app] keep your cached data under your allocated quota, your cached files will be some of the last on the system to be cleared when necessary. When the system is deciding what cached files to delete inside your app, it will consider the oldest files first (as determined by modified time)."
Long story short, they want app developers to better police their own apps in order to help the android system police itself. With cached data set on a per-app basis, routinely wiping out all cached data in every app at once may undercut the system's own enforcement abilities.
Note: I'm guessing.