My search for productivity tools that suit my platforms (linux @ home, windows @ work) and work philosophy (KISS, plain text preferred for transportability, not being locked into a particular tool or suite) continues.
I'm now using TiddlyWiki for general notes - one wiki file for home stuff and another wiki file for work stuff. I use minimal formatting in the tiddlers, but try to make good use of tags so I can track/trace ideas. I was using TiddlyWiki through Firefox but apparently a future release of Firefox (due in a month or two) won't allow TiddlyWikis to be easily saved. As a result I'm now using Pale Moon browser for my wikis and will probably migrate the majority of my web usage to/through it.
I used TiddlyWiki for to do lists for about 3 1/2 minutes until I found it too cumbersome for such duties.
Upon searching for information on plain text to do lists I came upon todo.txt which uses a fairly simple plain text format and set of conventions for tracking to do tasks. Here's an example:
x (A) 2017-10-19 2017-10-16 task description here +projecttag @contexttag due:yyyy-mm-dd
At its simplest a line could read buy milk, but (A) Buy Milk @shop provides both priority and context.
Even better is a cross-platform java program called jdotxt that simply reads/writes these plain text files and allows for searching by text and filtering by context and/or project. It also allows completed tasks to be archived to a separate done.txt file. The only downside I see with jdotxt is that it only allows for one todo.txt file to be in use.
My to do files are set up in Nextcloud and safely stored on my domain. This way it is readily accessible at home or work.
TiddlyWiki is not plain text, but simple and can read from and write to plain text files. hledger is an accounting tool that uses plain text files, and now jdotxt reads and writes plain text to do lists that can also read read/edited/understood in any plain text editor.
You've got to be happy with that!