As a practical matter, there are lots of applications that use GET for user submitted search data. Since GET requests encode user entered information into the URL and since the URL is typically found in web server logs and other tracking/history mechanisms, it is unwise to use GET for user-submitted data in applications that are concerned with privacy. However, it was not always considered unwise: REST advocates recommend GET when the underlying information representation doesn't change as a side effect of the request. Therefore, one might just as well say that logging of URL query parameters is the technical problem.

That said, when a "breach" has occurred is a legal distinction involving the control of information -- when protected data moves beyond those who have a duty to protect it. Saying that a particular technical approach creates breach is inaccurate.