"If their request to withdraw the excess was sent "on time" before April 15th this year"

For future reference, they were supposed to have made this request by March 1, to get a corrected W2.

"Also, how does it compare withdrawing excess from a pre-tax versus an after-tax 401k account? One of my clients is withdrawing her excess from her 401k Roth."

In both cases, any earnings are taxable, are distributed in 2024, and thus will be reported for 2024 by entering the 1099-R. There should be two issued: one for the earnings and one for the corrective distribution. The earnings will not be code P; they are taxable for the year distributed.

For after-tax (Roth), you enter basis.

Expand this for the codes for how to enter 2023:

https://accountants.intuit.com/support/en-us/help-article/form-1099-r/entering-form-1099-r-proseries...

Expand this topic for Making Excess 401(k) Taxable:

https://accountants.intuit.com/support/en-us/help-article/other-articles/entering-keogh-sep-simple-p...