Pretty sure you are correct, it was an optional rule. We just started playing again after a few years break. I was DM and catching up on the rules as we went along. My friends PC died after reaching 0hp. Just so happens in between then and the next session I came across the a deaths door rule and decided to implement it. Needless to say the player was a bit irked.It's my recollection that 2nd ed codified the Death's Door optional rule of death being at -10, with the 1hp/rd bleed, rather than at zero.