IRS Whistleblower Program Improvement Act of 2021 This bill modifies provisions of the Internal Revenue Code relating to whistleblower protections. Specifically, the bill revises the standard for review of whistleblower awards in the Tax Court to require a de novo review standard (currently, the standard is abuse of discretion); exempts whistleblower awards from reductions due to budget sequestration; establishes a rebuttable presumption that whistleblowers would suffer harm if their identities were disclosed in a Tax Court proceeding; modifies the Internal Revenue Service (IRS) whistleblower report to require inclusion of a list and description of the top 10 tax avoidance schemes disclosed by whistleblowers; requires the IRS to pay interest on whistleblower awards if not paid within one year of receipt of proceeds collected from whistleblower disclosures, allows the IRS to retain 3% of whistleblower proceeds (up to $10 million) to pay administrative costs of the whistleblower program; and allows payment of the attorney fees of whistleblowers regardless of whether the whistleblower award was paid through the mandatory or the discretionary whistleblower award program.