Came here to say the same thing, it's either Laurel or Oleander leaves that Pythia (the high priestess) either chewed or inhaled (their fumes). At least that's what I remember from high school and what most Greeks know. It's debated that the whole Oracle of Delphi thing could be a very well organized scam, but that's another discussion.
https://www.researchgate.net/figure/The-only-surviving-image...
Also, the ethylene theorists have a rebuttal. https://www.tandfonline.com/doi/full/10.1080/155636507014778...
It’s funny that neither paper attempted to just try the stuff and, you know, attempt mantic divination.
And there is some evidence for those laurel leaves (which were chewed) to be psychoactive. https://muse.jhu.edu/article/581067/summary
And, finally, that the oracle used lots (dice, basically) for divination. More randomness. https://www.journals.uchicago.edu/doi/pdf/10.1086/358831