"are made after filing dozens of client's 2020 tax returns in late Jan or early February, all of those returns would need to be amended."

Which can be done along with 1040X amendments required once the Corrected 2020 W2 is issued after the 2021 collection of the deferred Social Security amounts:

https://www.irs.gov/forms-pubs/form-w-2-reporting-of-employee-social-security-tax-deferred-under-not...

If the employer succeeds is collecting what is owed, of course.