Amecs

A Minecraft Fabric mod that allows to add key modifiers (e.g. shift or control) to every keybinding

Amecs (Alt-Meta-Escape-Control-Shift) allows you to redefine any keybinding in combination with modifier keys.
This is especially useful in conjuction with a lare amount of mods, so you don't need to find that one obscure key on your keyboard that isn't in use yet.

Additionally, Amecs adds a search bar to the Controls screen and makes keybindings non-blocking, so you can use multiple keybindings with the same key.

If you want to use key modifiers in your own mods' default keybindings, you'll want to use Amecs API which this mod is based on.
Amecs API also contains a bunch of other keybinding utilities like keybindings that trigger before any other key handling, so you can use e.g. while the user has some GUI open.

This mod is available under the CC0 license. Feel free to learn from it and incorporate it in your own projects.