LR sync file for an iMac in the Library/Cache folder. In troubleshooting a problem, Apple cleaned out the cache files to improve performance. That wiped out my sync file ... 40,000 images. Storing system files in the cache folder is a misuse of cache. It would be better to store the sync file in the same folder as the catalog. Less error prone.
Understood, but the Camera Raw Cache files are located in the same Library/Cache folder. They will also be deleted if that folder is purged. Moving it will prevent that from happening in the future. The deleted Camera Raw Cache files will be rebuilt as you view and edit image files in the Develop module. This will cause a small additional delay in the Loupe preview rendering until the new cache file has been built for each image.