The current cache system creates subdirectories directly in ~/Library/Caches. As a result, 3 subdirectories are created:

~/Library/Caches/Favicons
~/Library/Caches/FeedIcons
~/Library/Caches/Images

Such a cache system should be avoided to prevent interference between applications and to make it easier for users and the system to clean up caches. In fact, Apple's documentation includes the following statement:

Cache files should be placed in a directory whose name matches the bundle identifier of the app.
https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/MacOSXDirectories/MacOSXDirectories.html

That is, caches should be stored as follows:

~/Library/Caches/com.ranchero.NetNewsWire-Evergreen/Favicons
~/Library/Caches/com.ranchero.NetNewsWire-Evergreen/FeedIcons
~/Library/Caches/com.ranchero.NetNewsWire-Evergreen/Images