Unauthorized Spending Accountability Act of 2021 This bill establishes a three-year budgetary level reduction schedule for unauthorized programs funded through the annual appropriations process. Under the bill, a budgetary level is an allocation provided to the congressional appropriations committees under Section 302(a) of the Congressional Budget Act of 1974 by a congressional budget resolution or a deeming resolution. The schedule applies to programs included in the Congressional Budget Office's annual report listing programs that are funded through the appropriations process and have an authorization of appropriations that has either expired or will expire during the year. For the first year after a program's authorization has expired, the bill requires the budgetary level to be reduced by 10% of the funds appropriated for the program in the expiring fiscal year. The bill then requires reductions of 15% in the second and third years before terminating the program at the end of the third unauthorized year. Programs that are reauthorized during the three-year period are exempt from the budgetary level reductions if the reauthorization contains a sunset provision limiting the authorization of appropriations period to no more than three years. The bill establishes the Spending and Accountability Commission to review all mandatory spending programs and submit to Congress a legislative proposal to establish an authorization cycle for discretionary spending programs. The commission may recommend legislation to replace the budgetary level reductions required by this bill with reductions in mandatory spending. The commission's reauthorization schedule must limit reauthorizations to three years, include the budgetary level reductions established by this bill, and establish a mechanism for replacing the budgetary level reductions with reductions to mandatory spending programs. The House of Representatives must consider the commission's proposal using specified expedited legislative procedures.