Key components of the U.S. Internal Revenue Service tax processing code (e.g., the "Individual Master File" or IMF) are written in COBOL and IBM Assembly Language.

There is an ongoing effort to refactor as Java. This will ultimately take years and cost $100s of millions of dollars. There is still a small but shrinking team of graybeards who can actually maintain the code, which has to be reprogrammed every year to accommodate changes to tax code.

See, e.g., IRS IT Strategic Plan documents, publicly available.