Promotion and Expansion of Private Employee Ownership Act of 2021 This bill expands tax incentives and federal assistance for employee stock ownership plans (ESOPs) that are sponsored by S corporations. The bill provides additional tax incentives for ESOPs by (1) extending to all domestic corporations, including S corporations, provisions allowing deferral of tax on gain from the sale of employer securities to an ESOP; and (2) allowing a tax deduction for 50% of the interest received by a bank on loans to S corporation-sponsored ESOPs for the purchase of employer securities. The Department of the Treasury must establish the S Corporation Employee Ownership Assistance Office to foster increased employee ownership of S corporations. The bill defines an ESOP business concern for purposes of the Small Business Act as a business concern that was eligible for a loan, preference, or other program under such Act before more than 49% of the business concern was acquired by an ESOP.