Many older trucks have primitive ABS that is 2 wheel. A cable runs to a proportioning valve and is rigged to the bed. Since trucks are built to carry a load in the back, when they aren't, they brake majorly front heavy. When the bed reaches a certain angle moving upward the cable adjusts the valve to divert pressure rearward so that the rears don't unload and lock. Two wheel mechanical only ABS. Apparently they have a one channel, one sensor system too... didn't know that.
Cars, not so much... four it is.
"General Motors introduced the "Trackmaster" rear-wheel (only) ABS as an option on their Rear-wheel drive Cadillac models in 1971." - Wikipedia, ABS