That is refreshing. I wonder how much of the older PLC system has been replaced by modern techniques and UHF / Microwave frequencies.
100kc-200kc carrier line transmission by the utilities was state of the art in the 1940's.
Back in the 1970's I had an Elmer who was benefiting from the decommissioning of Westinhouse 136kc line carrier units - each 136kc "transmitter" had one 807 tube for the oscillator and six 807's for the amplifier in it. There were also these big-ass ginormous coupling capacitors for injecting and extracting the 136kc, as well as ginormous chokes for segmenting the system.
In the 1970's this stuff was being replaced by low-band VHF telemetry with whips antennas as strategic locations.
The most recent trend is fiber optics for SCADA. This goes back at least 20 years. They call it "Optical Ground Wire":
https://en.wikipedia.org/wiki/Optical_ground_wire