They were referring the fact that CRTs went the way of the dodo, and flat panels replaced them. This high frequency noise was a unique feature of the CRT.
CRTs lasted well into the 2000’s. I think the reference is to the fact that one loses the ability to hear CRT whine as one gets older, since the upper frequency limit of human hearing declines with age.
I suspect contravariant was making a joke that as you get older, your ability to hear high frequencies gets worse. After "a few decades or so" you likely won't be able to hear the flyback transformer squealing at 15 625 Hz any more.
At the end of my day at one of the post houses I was at around this time, I had to ensure all of the monitors were off. I could do it in the dark just by listening for this noise and know if any where on or not. Some of the reference monitors were never turned off though, and I had to ensure they had the correct signal routed to them to avoid burn-in.
I had an Atari ST in the living room and sometimes I would forget to turn the monitor off when I went to bed... but I was always reminded by the high pitch noise I could hear all the way in my bedroom.