Stop Settlement Slush Funds Act of 2021 This bill prohibits the federal government from entering into or enforcing a settlement agreement on behalf of the United States that provides for a payment or loan to any person or entity other than the United States. The bill provides exceptions to allow payments or loans that (1) remedy actual harm (including to the environment) caused by the party making the payment or loan, or (2) constitute a payment for services rendered in connection with the case or a payment that a court may order for restitution to victims in certain criminal cases or other persons in plea agreements. Government officials or agents who violate this prohibition may be removed from office or required to forfeit to the government any money they hold for such purposes to which they may otherwise be entitled. Agencies must report annually to the Congressional Budget Office about the parties, funding sources, and distribution of funds for their settlement agreements permitted by the exceptions in this bill.