> This isn't just about the metadata, you would be naive to think so. Proprietary apps, especially those that get automatically updated, will always have this loophole.

I would say most applications that goverment offices in Germany use to interact with citizens are proprietary. For example Outlook as a mail client. SAP is used in some administrations. Many cities and police departments have presences on Facebook and Twitter and communicate with people there. Nobody has any issue with using proprietary SMS apps or iMessage.

In fact, open source in administration is currently so rare that it is newsworthy.