see this because the proper form may depend on the type of plan
https://www.irs.gov/retirement-plans/terminating-a-retirement-plan
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-plan-terminations
depending on the type of plan the PBGC may need to be notified
https://www.pbgc.gov/about/pg/other/how-pension-plans-end
