How to enable modifier keys in X11.
https://emacsnotes.wordpress.com/2022/08/16/who-stole-c-c-and-possibly-other-keys-from-my-emacs/ looks interesting too.

X Keyboard Configuration Database
The goal is to provide the consistent, well-structured, frequently released open source of X keyboard configuration data for X Window System implementations (free, open source and commercial). The project is targeted to XKB-based systems.
https://people.uleth.ca/~daniel.odonnell/Blog/custom-keyboard-in-linuxx11 looks useful as well.

The unrelated article https://tonsky.me/blog/syncthing about Syncthing is interesting too. A great quote from https://tonsky.me/blog/phil-libin (not clear whether its from Phil or Nikita):
 The easiest emotional state to generate algorithmically is tribal outrage. It’s a simple and primal emotion. We, as the tech industry, have built a model that we make money when we piss people off. And everyone’s pissed off now, we’ve made a lot of money and people are like, what went wrong? Well, everything went exactly as planned.

Found on
See also http://www.rdesktop.org/ (dated?) and https://www.freerdp.com/ for open source UNIX clients.
The wiki page https://github.com/FreeRDP/FreeRDP/wiki/Keyboard has information about keyboard scan codes and remapping mechanisms in Windows and X11.

QMK (Quantum Mechanical Keyboard) is an open source community centered around developing computer input devices. The community encompasses all sorts of input devices, such as keyboards, mice, and MIDI devices. A core group of collaborators maintains QMK Firmware, QMK Configurator, QMK Toolbox, qmk.fm, and this documentation with the help of community members like you.
See also https://github.com/mihaiolteanu/mugur, an Emacs program to help configuring a QMK keyboard.

Main page is http://cowlark.com/cowgol/
It used to support the Apollo Guidance Computer used in the Apollo spacecraft, but I had to remove the code generator while rewriting the compiler and I haven’t reworked the AGC backend.
http://cowlark.com/2019-07-20-cowgol-agc/index.html
The author also ported the Fuzix operating system to the Raspberry π pico.
http://cowlark.com/2021-02-16-fuzix-pi-pico/index.html

His tool https://github.com/davidgiven/narcissus to remap keys might be interesting too.
So is his Forth system http://cowlark.com/fforth/

Map Caps Lock key to Escape key, or any to any key, on Windows systems.

Interesting software, such as
unifdef selectively removes C preprocessor conditionals. My version of this program now shipped by all the BSDs (including Mac OS X) and is used by the Linux kernel build system.
regpg safely stores server secrets using gpg, so you can keep them in version control.
picoro - tiny coroutine implementations in pure C. I wrote an accompanying article, coroutines in 20 lines of standard C.
Counting the days - tiny routines for converting Gregorian dates into linear counts, like Julian day numbers or Unix time_t. The date of the count - a small routine for converting linear day counts into Gregorian dates.
https://dotat.at/@/2016-04-22-synergy-vs-xmodmap-fight.html Why you can't use xmodmap to change how Synergy handles modifier keys
His log of links, https://dotat.at/:/ which I found on the TUHS mailing list is also worth reading. He explains a bit about this log on http://dotat.at/.

How to choose and configure keyboards for efficient typing. Found Wed Dec 21 18:38:57 2016, rediscovered Tue Aug 28 01:08:05 2018.
The blog has other intersting articles, e.g. https://stevelosh.com/blog/2018/08/a-road-to-common-lisp/, https://stevelosh.com/blog/2013/09/teach-dont-tell/

Information about keyboard handling and rebinding in Windows.