www.jdsupra.com/legalnews/irs-passport-revocation-260-000-and-89090/
www.irs.gov/newsroom/irs-urges-travelers-requiring-passports-to-pay-their-back-taxes-or-enter-into-payment-agreements-people-owing-51000-or-more-covered
See also:
goo.gl/WWScuK
goo.gl/5Z25zq
www.duanemorris.com/alerts/us_citizens_federal_tax_debt_may_be_unable_travel_abroad_0419.html
Update:
bambridgeaccountants.com/us-expat/2020/10/29/americans-continue-giving-up-citizenship-at-record-levels