Save Social Security Act of 2021 This bill modifies the requirements for Social Security payroll taxes and benefits to apply Social Security payroll taxes to annual income above $400,000, include earnings above $400,000 in the benefit formula, and increase to $100,000 the income threshold above which a portion of a taxpayer's Social Security benefits is included in gross income. (Under current law, Social Security payroll taxes apply to the first $142,800 of income in 2021 and a portion of benefits is included in the gross income of individuals with certain income that exceeds specified thresholds, depending on the individual's filing status.)