Thanks to Mike for finding this article.
As reported by the National Review, a U.S. Tenth Circuit Court of Appeal ruled that it is ok for the U.S. Department of State to revoke a U.S. passport (to be used e.g., for international travel) if the IRS has certified that the person owes more than $50,000 in federal taxes.
As the Court says: “…the passport revocation serves only to incentivize repayment of the tax debt.”
See the July 20, 2021 ruling.
Do US persons have a right to international travel?