Corporate workplace spying on employee communication became prevalent in the 90s to prevent discrimination lawsuits. This predates what we think of as our contemporary "surveilance capitalism". There were a series of court decisions that enshrined this practice in law and since then the limits of proscribed speech have expanded.

"The Unwanted Gaze" is a good read on this subject. <https://www.penguinrandomhouse.com/books/157389/the-unwanted...>