From the Daf Review:
"When, after Ula had informed Rava that he stayed overnight in Kalnevo, the latter queried the fact that he had mentioned the name of a god, he replied with a statement of Rebbi Yochanan who ruled that one is permitted to mention the names of idols mentioned in Tanach which have since become obsolete."
Not saying this is wrong, just wondering what is the source that this is the reason for the heter?
1) The source is the Sefer Yere'im #75, by one of the Rishonim, Rabeinu Eliezer of Metz. He writes that since the Torah mentions these names, this must mean that they had certainly been abolished by their owners. The Yere'im writes that for the same reason that the Torah mentioned these names, we are also allowed to mention them.
2) The Gilyonei ha'Shas (by Rav Yosef Engel) on the Gemara here quotes the Midrash Rabah Shemos 30:9, which teaches that Hash-m tells the Jewish poeple to do what He does Himself. This means that Hash-m, so to speak, observes the Mitzvos. If it would be forbidden to mention these names, Hash-m would not have mentioned them in His Torah. This proves that the idol-worshippers must have rendered these idols obselete by the time that the Torah was given.
Kol Tuv,
Dovid Bloom
Why is this on sanhedrin??
The Mishnah in Sanhedrin 60b discusses the punishment that the Sanhedrin gave for somebody who worshipped idols. The Mishnah continues and states that one is not allowed to vow in the name of an idol. The Gemara 63b discusses further about not mentioning the name of Avodah Zarah. One may not say to a friend, "Wait for me next to such and such an idol." However, Rebbi Yochanan told us that if the name of the Avodah Zarah is mentioned in the Torah, one is also allowed to mention its name in everyday speech.
Kol Tuv,
Dovid Bloom