This would be an interesting feature to have it it really worked, but I am skeptical whether it can be implemented robustly enough to be anything more than a giant pile of false positives. Tables get passed around in all sorts of ways (as modules), passed to different scopes, copied to new structures, accessed by meta-methods and raw accessors, serialized, the number of keys in a table can be relevant without direct access to a specific key, etc. Understanding when a field was really used or unused might be hard to implement.