I heard about it from my doctor who suspected I had it. He didn't call it TOS but described TOS and said that people sometimes need their rib removed. He sent me for an MRI and an EMG/NCV. Three weeks later - confirmation came with a positive EMG/NCV. This was 8 years ago.

Like Coop42 said, the information is becoming more readily available. And more and more doctors seem to accept it or at least acknowledge the existence of TOS.