fvwm, customized to be basically a kiosk-like desktop with a few large buttons for different use cases (each one a virtual screen).
Currently on debian, don't remember which distros I used initially. Probably started on slackware.
For email, mutt & emacs. Although both are heavily customized to have simple and minimal commands available, so a regular mutt or emacs user would be a bit lost.
I can't stress enough the power of customizable software! The ones on list list (fvwm, mutt, emacs) are prime examples of excellence on this front.
There is nothing worse than "opinionated" software that doesn't allow extensive customization. Even worse, when they change interfaces every time a new PM joins the company. For older people, any change in interface is a huge setback. There is unappreciated value in software than can be fully customized and then left alone.
The one and only change I made to the setup over the years was to remove the connect/disconnect icons for the (dialup) modem when I upgraded them to DSL and even that small change took a months before they stopped asking about it.
The most problematic part of this stack has been the browser, which I can't keep static and has changed a bunch of times in the last ~28 years. To the extent possible I configure the toolbar to be minimal. But the basic functionality of clicking on home button and back button hasn't really changed in the last three decades and that's basically all they need, so it's been manageable.