Because your employer excluded the excess elective deferral from the taxable amount in box 1 of your 2021 W-2, you have to add it back to income on your 2021 tax return, so it is taxed on your 2021 tax return and does not create after-tax basis in your 401(k). The same money will be taxed again by reporting it as a regular distribution when eventually distributed from the 401(k). If that distribution occurs in 2022, it must be included on your 2022 tax return, otherwise that distribution must be reported on the tax return for whatever later year in which the distribution occurs.
