There are a ton of specially formed oil blends with high ZDDP that are intended for "race use" and don't exactly need certification from the EPA/DOT or whomever it is they go through. However, those specific oils don't come without a heavy price.
When analyzing and before the EPA started pushing for greener oils became a thing, people went crazy buying oil like Rotella T because it was cheap, easily found local, and high in ZDDP content. Mobile 1 even used to be a great oil with high ZDDP, but that changed maybe 4-5 years ago.
So you ask what they used before T6? People "in the know" had a lot of choices, but people didn't start making a stink about ZDDP until recently when the EPA started giving a fuck about clean oil.