Presidential Allowance Modernization Act of 2021 This bill revises the compensation provided to a President who leaves office after enactment of this bill or such a President's widow or widower. Each such President, excluding a President removed from office through impeachment, shall receive (1) an annuity of $200,000 per year for the remainder of the President's life, and (2) a monetary allowance of $200,000 per year. Such allowance shall be reduced by the amount the President's earned income exceeds $400,000. These monetary amounts are subject to a cost-of-living increase. The widow or widower of each such President shall be entitled to receive $100,000 per year, payable monthly, if such individual waives the right to each other annuity or pension to which the individual is entitled. This amount is subject to a cost-of-living increase.