I can't find any Quick Look cache data in macOS Mojave beta
In Mojave, quicklookd stores the information in the same private folder hierarchy:

/private/var/folders/&lt;id&gt;/&lt;random&gt;/com.apple.QuickLook.thumbnailcache


as in High Sierra.

To list the exact folder name used in Mojave (this procedure also works in High Sierra), open a Finder window, select a document or application, press space to fire up Quick Look, launch Terminal and run:

sudo lsof -c quicklookd


lsof -c displays which files and folders are in use by a specific process. 
The result will look similar to this (the red circle highlights the private folder):



The folder can't be accessed, neither by a standard user (even with sudo) nor by an administrator:


As you posted in your comment, the reason why you can't see its contents is because it is protected by System Integrity Protection. That's new in Mojave.