The IRS's historical position was that, with a few exceptions that wouldn't be pertinent here, discretionary plan amendments could not be effective for plan years before the year in which they were adopted. SECURE Act 2.0 partially reversed that policy. New IRC §401(b)(3) provides that an amendment that increases benefits may be effective as of any day in the preceding plan year, so long as adoption is no later than the extended due date, including extensions, of the employer's income tax return for the taxable year within which ends the plan year that contains the effective date. Since adding tips to the plan's definition of "compensation" can only increase benefits, it is unobjectionable.
I agree with the previous comments about the practical difficulties of taking tips into account, except for tips that are reported on employees' W-2's.