[bostik]

In the non-optical domain, that used to be a feature in the most expensive cable measuring gears (to the tune of €25k about 20 years ago).

I had the pleasure of using one of the devices that had that when I wired my old house. Borrowed the kit from ... an institution that allowed me to get their gear for a weekend. (Yes, I knew people in there.) Having the device tell me that a given strand in a CAT5e cable was faulty at <this many meters> from my location made it surprisingly easy to detect where I'd messed up.

[colanderman]

Many (low-cost) Mikrotik routers have this feature built in now: https://wiki.mikrotik.com/wiki/Manual:Interface/Ethernet#Det...

[ahartmetz]

I first saw the feature on a motherboard integrathed Ethernet chip over 10, possibly over 15 years ago.