Mainframes are a good place to look. IBM VM dates back to 1972. IBM's other mainframe operating systems trace their lineage back to OS/360, which is 1966. The

Mainframes are a good place to look. IBM VM dates back to 1972. IBM's other mainframe operating systems trace their lineage back to OS/360, which is 1966. The XEDIT editor is 1980. I believe codebases for all of the above are still in active development and are certainly in active use.