I learned about it at least a few years ago watching a history of video game speedrunning video (don't remember the specifics from the video or which video it is, sadly). At least 6 years ago, the speedrunning community used the mains hum to prove that a video was spliced together, rather than a continuous stream.

Also, clearly the creator of this page has been recording the mains hum for at least 6 years, so presumably they also knew about it.