Stopping Illicit Trafficking Act of 2021 or the SIFT Act of 2021 This bill adds five fentanyl analogues and the entire category of fentanyl-related substances to schedule I of the Controlled Substances Act. A schedule I controlled substance is a drug, substance, or chemical that has a high potential for abuse; has no currently accepted medical value; and is subject to regulatory controls and administrative, civil, and criminal penalties under the Controlled Substances Act. The bill also creates a process to remove from schedule I certain fentanyl-related substances that do not have a high potential for abuse.