Emergency Pension Plan Relief Act of 2021 This bill modifies the funding rules and provides financial assistance for certain pension plans that are underfunded or insolvent. First, the bill expands the authority of, and provides funding for, the Pension Benefit Guaranty Corporation (PBGC) to provide special partition assistance to a multiemployer pension plan that is insolvent or at risk of insolvency. The bill expands eligibility for partition assistance, provides funding for a plan to reach a projected funded ratio of 80% over a 30-year period, and does not require a plan to repay such assistance. The bill further permits a multiemployer pension plan to elect to retain its funding zone status from the previous year for either (1) the first plan year beginning during the period from March 1, 2020, through February 28, 2021; or (2) the next succeeding plan year, as designated by the plan sponsor. A plan may also extend by five years the funding improvement or rehabilitation period if the plan is designated as in endangered or critical status for a plan year beginning in 2020 or 2021. A plan in critical and declining status may not suspend payment of plan benefits. Additionally, the bill adjusts the minimum funding standards for a multiemployer pension plan to account for investment losses and other losses related to the COVID-19 (i.e., coronavirus disease 2019) pandemic and modifies the PBGC guarantee formula to increase the maximum potential benefits under a multiemployer pension plan. Finally, the bill makes changes with respect to single employer pension plans, including revising the amortization rules and extending and modifying the pension funding stabilization percentages.