Rebbetzin:
The Pri Migadem s. 453 Mishbitzos Zahav ss.1 and a”a s. 464 ss.1 writes that potatoes are mutter IF it is not in a place that prohibit the. (This implies that such places exist!
No, it merely implies that the Pri Megodim thought such places might exist, somewhere in the world. Perhaps he even heard rumors of such places. But he had no first-hand knowledge of them, because there weren’t any.
The Nishmas Adam s. 20 that the custom was not to eat potatoes except during a great hunger the Bais Din convened to be mattir it to avoid danger and extreme hunger.
Again, he was not speaking of his own knowledge. He had never seen such a place, or any evidence of its existence. He heard rumors, that is all.
That is the history of this strange myth. Nobody ever writes that here we ban them, let alone that I ban them. It’s always someone else, far away, where I’ve never been.
It is brought in Alef that Rebbe Yehoshua from Belz was makpid not to eat potatoes.
What is “Alef”? In any case, all kinds of people don’t eat all kinds of things on Pesach , without thinking they’re forbidden. Belzer don’t eat carrots, but they don’t asser them. Lubavitchers don’t eat radishes, Oberlanders don’t eat fish, Egyptians, who do eat kitniyos, don’t eat chickpeas. So it doesn’t surprise me that someone didn’t eat potatoes, but it would greatly surprise me if he held they were kitniyos.
One sefer says that we are all fortunate that there were no potatoes in times of Geonim for if they would have them then, they would surely prohibit them as kitniyos because of their ability to produce flour.
That is pure speculation. “One sefer” can say whatever it likes about what could have been in some alternative universe, but in this universe nobody ever prohibited potatoes. Nobody.