As software updates, older versions become vulnerable. When Cheetah Mobile updated Clean Master to newer versions, they likely patched security holes and changed the licensing algorithms. Users searching specifically for 6.6 are often looking for a version of the software that was successfully "cracked" in the past. They are seeking a known vulnerability in the licensing system that has been widely distributed.
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However, if you are looking for a "Pro" license key, it's usually to unlock premium features such as: Intelligent Auto-clean: As software updates, older versions become vulnerable