What is it, why hasn't anything replaced it, and what would it take to retire it?
How do you define old? The oldest initial release or the last update?
JOE [0] for editing directly in terminal (Linux, macOS)
Somebody gave me Slackware Linux [1] floppies in 1994 and schooled young me on worlds beyond MS-DOS and Windows 3.11 For Workgroups; I had accidentally discovered MS-DOS Editor [2] by being frustrated one day and typing edit into the command line (while muttering "HOW DO I EDIT A FILE?", barely understanding what a file was in the first place), so I was shown that JOE was like edit but also way better. The UI and keybindings ended up burned into muscle memory, so I just kept using it, and still do -- and there's basically no point in ever replacing it now or starting to use emacs/vi/etc instead lol
[0] https://en.wikipedia.org/wiki/Joe%27s_Own_Editor
1995 for me and Slackware but I had the tiniest advantage in having used WordStar in DOS quite a bit beforehand. JOE still remains my preferred editor as a result.
Well it's not quite a 'software tool' but where I work we still sometimes have to handle a deck of column binary punched cards.
Notational Velocity's last update was in 2011. Still works like a charm 15 years and 14 macOS releases later.
Circuit Maker 2000, for designing descrete circuits. I've tried to move to LTSpice a number of times, but it's interface is less intuitive. CM200 has a really clean UI for design and simulation. Doesn't run as well on Windows as it used to, but still good enough
vi. Fifty years old now. It's a defacto TUI text editor on any embedded Linux system I work with and I can count on it to always be there. No replacement comes close, or has a smaller footprint in storage.
dc
Not sure exactly, but probably either Emacs or Bash.
% vim --version
VIM - Vi IMproved 5.8 (2001 May 31, compiled Jun 1 2001 23:17:28)
No need to retire it, because it just works..