"The changes made by [ARPA] will affect minimum funding requirements generally. The possible retroactive application of these changes, and the elections permitted with respect to their application, raise a number of issues ... There are also decisions for sponsors about whether to apply the interest rate changes in 2020 and 2021 for all purposes (including benefit restrictions) or just for purposes of minimum funding. [Notice 2021-48] generally provides wide latitude on the treatment of these issues."