I've a run of around 60 meters of old telephone cables (made out of copper, 4 wires) and I can get 100Mbps on those reliably. I used the old telephone infrastructure on the building to pass network from an apartment to the basement that way.

Not to spec on ethernet on any way, not even twisted pairs but they do work. Unfortunately I can't replace the run with a proper Cat6 cable because there's a section that I can't find where it goes to, it just disappears on a floor and appears 2 floors bellow it inside the main telephone distribution box.

On the distribution box (that is already on the basement of the building) I'm plugging into the LSA connector that goes to the apartment:

The black box you see there is a mikrotik gper that is essentially a PoE switch with only 2 ethernet ports so I can get over the 100m limitation of ethernet. I'm running a cat6 cable from there on metal cable trays for about another 90 meters until it reaches a storage unit 2 floors bellow ground.

Here's a ping test from a machine sitting on the storage box to the router on the apartment:

The router reports this as a 100M full-duplex connection:

If anyone wants to try a setup like this, or just extend ethernet > 100m, it also worked fine with a cheap 5$ 100M switch from Aliexpress and a PoE injector + splitter.

However I eventually got the mikrotik gper for free so I decided to replace it because it should be more reliable.