Guys I am begging you not to automatically take at face value two or three ancient descriptions of a particular herb's abortifacient qualities. Ancient medical authorities by and large did not know what the fuck they were talking about.
I cannot find any peer reviewed sources linking the "discovered" plant to the ancient one, nor can I find any scientific sources testing the supposed abortiofacient qualities of the discovered plant.
I did find this blog by a classics MA debunking the myth of silphium the magical abortifacient.
And this article points out that the presumed abortiofacient qualities of silphium would not be relevant for an overharvesting extinction because Roman law at the time (and fifty years before!) privileged having many children.
Silphium might have been part of the fennel family, which does have some minor abortiofacient qualities. If it was a type of fennel then yes, sure, might have been good for that, but the ancient sources also say it was a treatment for epilepsy, dog bites, and respiratory illnesses. It was not specifically held up by the ancient sources as solely or even primarily a medicine for ending pregnancies. The article I linked there even admits that one of the passages they use to highlight the ancient appreciation for silphium could just be referencing the fact it was associated with the region of Cyrene.
It was not overharvested because it was an effective abortiofacient. It was overharvested because it was delicious. The ancient sources speak to this.
The reason we think it's an abortifacient above everything else is because of a historian named John Riddle, who made some rather expansive claims about it. That escaped into listicle land.
We HAVE safe, effective abortifacients. The problem isn't that we don't have enough. The problem is that we don't have access to them. Pliny isn't a fucking pharmacologist.