The herb you're describing is Silphium, and the idea that silphium was harvested to extinction because it was an abortifacient is commonly expressed on reddit, but is likely not historically sound. It had abortifacient properties, but it was also prized as a flavoring in food and had an (allegedly) incredibly pungent flavor to the point that wealthy people were willing to pay a fortune for very small amounts.

The market for silphium was kind of like the market for saffron or truffles today (truffles are also on the verge of extinction for the exact same reasons BTW). Ancient cookbooks were full of advice on how to improvise a silphium replacement if you couldn't afford it, or how to "stretch" your silphium by putting it in a jar with pine nuts which would pick up the flavor (sort of like truffles in oil today).

So while it was potentially useful as an abortifacient, it was primarily used in cooking, but was already quite rare in antiquity. There were many other more common herbs that also functioned as abortifacients (e.g. pennyroyal, ergot), many of which are still in use as abortifacients in parts of the world today, but there was nothing else that really tasted exactly like silphium (again according to ancient cookbooks) which is the real reason it was harvested to extinction.