We are very excited to see you next week!
The full information about the 7th OpenSky Workshop in Zurich, Switzerland (November 2019) can be found on the new dedicated website https://workshop.opensky-network.org
Tl;dr: Not that we know of. At least not deployed anywhere outside a lab. Some proposals have been floated, however.
This post was inspired by the recent report by the US Congress Government Accountability Office on ADS-B, which was discussed in many articles around the web, such as this one, for example:
Let’s have a look at the source, these are quotes from the GOA’s actual document, citing some of the work from contributors here at OpenSky: 
Yes, they do, at least partially, but there are massive differences between countries.
Let’s add some data to this popular question, shall we? This is from our recent paper at the. We collected ADS-B and Mode A/C/S data from over 6000 aircraft operated by militaries all over the world (with a strong focus on Europe/the US) using the . The key plot to answer the question is the following: