True Up Contributions

If a participant didn't defer the maximum they could in one plan year, is it allowable to allow them to make a sort of true up contribution and give them the opportunity to make additional contributions for the prior year?

Nope. 

I am confused by which years and what maximum(s) you are asking about. Absent an unexpected clarification, I agree with justanotheradmin.

There is a special catch-up available in 403(b) plans that is designed to allow participants to make up for some years in which they didn't maximize their contributions. I don't deal with 403(b) plans enough to feel comfortable explaining it here, but you can research it if you're interested.

There is nothing along those lines for 401(k) plans however.

On 1/25/2024 at 11:13 AM, C. B. Zeller said:

There is a special catch-up available in 403(b) plans

And 457(b) plans as well.

A deferral limit catch-up that looks to a span of years (with other conditions) is in each of Internal Revenue Code § 403(b) and § 457(b).

A § 403(b) participant who’s 50 and has 15 years with the qualified organization might defer up to $33,500 [2024].

A § 457(b)(3)(A) catch-up might allow a deferral up to $46,000 [2024] for some participants.

Complete explanations of a section’s catchups are in 403(b) Answer Book and 457 Answer Book, published by Wolters Kluwer and available in its VitalLaw® products.