Given the rate of government recognition and response to issues, I would classify the last 30 years as new. However, I'm not sure about the private assertion. The first cell phones were analog and anyone with a scanner could listen in. Any agency with a court order can listen to digital cell phone calls. Email, SMS chats, etc. are all accessible via court sanctioned search permits. End-to-end encryption has enabled truly private communication (ignoring NSA high compute capabilities) and that has not been available in a widespread way until the last few years. I'm not referring to computer knowledgable HN types.

I'm not aware of a study looking at that particular correlation, but there is plenty of data available on the problem itself. I wouldn't be surprised if such a study exists though. I am no expert, but I have read reports from what I at least consider reputable sources. Its an easy search.