Proprietary encryption key
What if the key was in a book? It would have to be protected by free-speech, which makes it uncensorable.
What if the key contents were used as hex values to make a flag? Would you censor a flag too?
No such thing as "proprietary encryption keys" exist.
*Not all of the app data, only the app data in the internal user-accessible storage. The data in the /data partition will not be backed up. Most app settings are stored in /data.