How to regain warnings about permanent version storage?
For apps that are not sandboxed

Example: 

To regain warnings about permanent version storage for Apple Pages

Quit the app, then a one-line command in Terminal: 

defaults write ~/Library/Preferences/com.apple.Pages NSDocumentSuppressTempVersionStoreWarning -int 0


– in the Preferences area that's familiar to users of Snow Leopard and less. 

For apps that are sandboxed

The operating system uses a different approach to containment of some preferences: 

~/Library/Containers/&lt;bundle_id&gt;/Data/Library/Preferences

Example:

To regain warnings about permanent version storage for Apple Preview

Quit the app, then a one-line command in Terminal: 

defaults write ~/Library/Containers/com.apple.Preview/Data/Library/Preferences/com.apple.Preview NSDocumentSuppressTempVersionStoreWarning -int 0


Thoughts

I should have thought first of ~/Library/Containers when, in the question, I observed –


  com.apple.Preview.plist is not recently modified 


– but I didn't realise what was going on until after I used fseventer to visualise writes for a different user of the OS. 





References

App Sandbox Design Guide: Migrating an App to a Sandbox

A visual record of me finding the relevant key in the .plist for Pages

A visual record of me finding the relevant key in the .plist for Preview