A standard numpad comes with 17 keys (including the num lock key) to remap to whatever you want and you can combo them with shift + ctrl + alt.
It’s also not just “custom icons”, it’s a live display — e.g. in the top image of the linked post, you can see a display on one of the buttons showing (presumably) an upcoming calendar event.
You can do this with custom key mappings on all major operating systems. Basically you can open a program, open a folder, run a script or do anything you can run from the command line (including running multi-step operations).
On Windows there's AutoHotkey. It sounds like it might be complicated but it's not. For example this single line of code `#+w::Run chrome.exe -incognito` lets me press Win + shift + w to open an incognito Chrome window and `#n::Run, explorer.exe \\wsl$\Ubuntu-20.04\home\nick\src\sites\nickjanetakis` opens my personal blog's directory inside of an Explorer window (default folder viewing app) with Win + n. I wrote a blog post[0] about using the basics of AutoHotkey and there's similar tools for Linux and macOS at least in the sense of being able to assign any action to a hotkey.
[0]: https://nickjanetakis.com/blog/remap-and-set-global-hotkeys-...
If so you can do that with numpad re-mappings too. You could have 17 specific key mappings depending on which app is in focus or you can arbitrarily activate a different specific set of mappings.
It's not going to be as intuitive since there's no visual icon to display but the functionality is there. AutoHotKey could also make it easy to show which "folder" is active by showing a little tooltip when you switch folders.
It sounds like the real appeal to it is the visual icons but the trade off is needing more desk space and reaching for buttons that are not on your main keyboard (+$125 to buy it).