[39b middle] Rashi [quoting Chulin 105]: Mayim Achronim [washing one’s hands after a meal before Benching (saying grace)] is obligatory, due to the precautionary measure of Sodomite Salt being on ones hands (which is extremely small and potentially blinding). Tosephos (Brachos 53): The problematic salt is no longer around now-a-days so there is no longer a prohibition. Even though a Rabbinic law was made, perhaps it does not apply because it was only established in places that had this problem. Currently, since no places have this issue anymore, the law is not uprooted but just simply does not apply.
E’s Lesson: We must remember to always protect, safeguard, and strictly fulfil Rabbinic law. We may never dismiss a decree, and rely upon our sages to know when and where they are applicable. The Torah specifically commands us to follow our Rabbis and Teachers, so that every time we do so we are in effect fulfilling a direct command from Hashem.