On September 6, 2000, the Treasury department published a final rule to amend the 1988 rule.
Under that amendment, a plan sponsor may amend its plan to remove an optional form of benefit (including an optional annuity) if the plan provides a single-sum distribution form otherwise identical to the optional form of benefit eliminated. Such an amendment must not apply to a participant with an “annuity starting date” earlier than the 90th day after “the participant has been furnished a summary that reflects the amendment and that satisfies the requirements of 29 CFR [§] 2520.104b–3[.]” (I’ve simplified those explanations, and omitted some conditions.)
For the details: Special Rules Regarding Optional Forms of Benefit Under Qualified Retirement Plans [final rule], 65 Federal Register 53901-53909 (Sept. 6, 2000), https://www.govinfo.gov/content/pkg/FR-2000-09-06/pdf/00-22668.pdf.
Also, the Treasury department published further amendments in 2004, 2005, and 2006.
https://www.govinfo.gov/content/pkg/FR-2004-03-24/pdf/04-6220.pdf
https://www.govinfo.gov/content/pkg/FR-2005-08-12/pdf/05-15960.pdf
https://www.govinfo.gov/content/pkg/FR-2005-09-13/pdf/05-17959.pdf
https://www.govinfo.gov/content/pkg/FR-2005-09-27/pdf/05-19222.pdf
https://www.govinfo.gov/content/pkg/FR-2006-08-09/pdf/E6-12885.pdf