Excess not distributed. If you don't take out the excess amount, you can't include it in the cost of the contract even though you included
it in your income. Therefore, you're taxed twice on the excess deferral left in the plan—once when you contribute it, and again when you receive it as a distribution (unless the excess deferral was a designated Roth contribution. You must include the excess deferral in your income for the year of the deferral. File Form 1040 or 1040-SR to add the excess deferral amount to your wages on line 1.
To the extent that a corrective distribution is not made within the correction period, the excess deferrals may not be distributed until a distribution is otherwise permissible under the terms of the plan, or the distribution is necessary to avoid plan disqualification under IRC Section 401(a)(30). Reg. Section 1.402(g)-1(e)(8)(iii) provides that distributions of excess deferrals after the correction period may be distributed from a 401(k) plan only when permitted under IRC Section 401(k)(2)(B).
Under that Reg the excess is treated as an employer contribution so you have no tax basis in it and that's why it is taxable when withdrawn (ROTH a/c exception)
so yes you need to amend 2021 to report the excess contribution as income.
there is no penalty for an excess 401(k). just that you will have income twice. once in the year made and eventually when you withdraw it,
here are the rules for withdrawing the excess late IRC 401(k)(2)(B)
(B)under which amounts held by the trust which are attributable to employer contributions (the excess is regarded as an employer contribution) made pursuant to the employee’s election—
(i)may not be distributable to participants or other beneficiaries earlier than—
(I)severance from employment, death, or disability, - thus the excess would have to come from your former employer's plan
(II)an event described in paragraph (10), such as plan termination
(III)in the case of a profit-sharing or stock bonus plan, the attainment of age 59½,
(IV)subject to the provisions of paragraph (14), upon hardship of the employee,
(V)in the case of a qualified reservist distribution (as defined in section 72(t)(2)(G)(iii)), the date on which a period referred to in subclause (III) of such section begins, or
(VI)except as may be otherwise provided by regulations, with respect to amounts invested in a lifetime income investment (as defined in subsection (a)(38)(B)(ii)), the date that is 90 days prior to the date that such lifetime income investment may no longer be held as an investment option under the arrangement,
