If the plan’s governing documents do not constrain what the plan’s administration might responsibly choose:
The notice of proposed rulemaking C.B. Zeller points to includes a transition rule:
“For purposes of paragraph (b)(2) of this section [calling for forfeitures to be used no later than 12 months after the close of the plan year in which the forfeitures “were incurred”], forfeitures incurred during any plan year that begins before January 1, 2024, will be treated as having been incurred in the first plan year that begins on or after January 1, 2024.”
And: “Taxpayers . . . may rely on these proposed regulations for periods preceding the applicability date.”
If the plan TH 401k describes is on calendar plan years, the plan’s administrator might, with its lawyer’s advice, consider using the forfeitures from 2023 and 2024 during 2025.
Although the notice of proposed rulemaking states only a proposed rule, not a final or interim rule, a practitioner might suggest her client follow (at least) the proposed rule Why? The Treasury department explains that the proposed rule is based on decades-ago IRS interpretations. Until there is a Treasury rule, an IRS examiner might assume that the IRS’s interpretations are the correct interpretations of the Internal Revenue Code’s tax-qualification conditions.
As ratherbereading suggests, a plan’s administrator might read the plan’s governing documents and seek to follow them as nearly as one can.