"Rev. Proc. 2015-28 updates the safe harbor correction methods and examples in Appendices A and B to Rev. Proc. 2013-12 to provide alternative correction methods associated with missed elective deferral for eligible employees subject to an automatic contribution arrangement.... Notably, this safe harbor 'sunsets' on December 31, 2020, and will not be available for failures occurring after that date unless the IRS decides to extend it."