Assembly Language
The IRS is still running the same code, which was written nearly 60 years ago. Most of it is in the Assembly programming language, which the IRS itself has described as “antiquated” and “inflexible.” Worse, the number of programmers who can understand and maintain the code behind the Individual Master File (IMF) dwindles with every passing year. According to the Government Accountability Office, the IMF and its business counterpart (the BMF) are the oldest computing systems used by the federal government.
https:// www.bloomberg.com/view/articles/2018-04-17/the-irs-computer-system-is-the-oldest-in-the-government