The actual folder is "/System/Library/Desktop\ Pictures", but it's protected on newer systems. It consumes 414MB on my machine, which is 0.3% of a 128GB drive. Doesn't seem worth the trouble to me.
BTW, I found the path by double-clicking the folder as seen in Desktop & Screen Saver, then dragged that selected Finder folder to a terminal window.
No, this is the wallpapers folder used to store all the default macOS wallpapers, not a cache folder, don’t delete it (if you don’t want to delete all the macOS stock wallpapers).
BTW, I found the path by double-clicking the folder as seen in Desktop & Screen Saver, then dragged that selected Finder folder to a terminal window.