Look, as a fairly rational conspiracy theorist, I recognize the importance of evidence, but in cases like this, it's very unlikely unless a leak happens for you to ever get it, and hence instead of looking for deductive evidence you can never rely on getting, you have to use inductive logic instead, and the difference between the two logical methods (inductive/deductive) is an important distinction that isn't made enough.
Do we have hard evidence that KDC was spied on specifically? No. Do we have inductive reason to believe it's probable? Very much so.
https://www.theguardian.com/world/2013/nov/20/us-uk-secret-d...
If there was evidence of such loopholes, people could demand change from their representatives. Without proof, I guess the only thing someone can do is repeat it in any IC-related thread on HN to get some easy karma points (Seens to work well!).
This is often repeated, but I don't believe any evidence has been published to indicate this is happening.