It's quite difficult to find software older than Emacs in widespread use. Emacs is one of the "original" software, the first GNU software for which the GPL was created. It competes with vi, whose direct lineage has been broken (nvi and vim are reimplementations).

> It's quite difficult to find software older than Emacs in widespread use.

Agredd! Aside from the basic Unix commands (ls, cat, etc), the only substantial program that I use today that I also used in 1991, is emacs.

Well, and perl. I don't write much perl these days, but I have tons of perl scripts accumulated over the decades that I use daily so that counts as using it.

Third oldest for me would be mutt, which is still my primary email client. But I only switched from elm to mutt in.. not sure maybe 1996?

vi recently received an update. C23 compliance and some unicode fixes. it's a feature complete editor!

One advantage of vi is that even though emacs is available on all those systems, vi is actually installed on all of those, setting aside windows. If you know rudimentary vi you can walk up to any of those machines and edit a configuration file well enough to work.

mg is installed on macOS by default and is basically diet vanilla emacs. Works fine for quick edits.

vi isn't even in the same neighborhood in this regard. You can and should learn vi (and ed!) in one afternoon even if you never use them. Whereas Emacs isn't something you just learn in an afternoon. (And vim is not the same as vi)

That is a separate discussion from whether you should use ed or vi normally. You learn the "fallback" with the hope of never being forced to used them.