"Occasionally, an employer has to make a late makeup 401k contribution for a prior year, so an entry labelled D20, on a 2021 W-2 form, would indicate that such a makeup contribution was made for the year before (i.e. 2020). "
That not how this works. Code D reports employee elective deferrals. If the employer failed to include the correct code-D amount on the 2020 W-2, the employer must correct the 2020 W-2.
The D21 indicates a USERRA make-up contribution. Since it was made in the same year as the year of the W-2, the "21" is redundant and can be omitted. If there is a separate code-D amount with no year designation, the two amounts can be combined. (If the USERRA amount was instead for a prior year, the USERRA amount should be omitted.)
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-userra-and-sscra
