In the past I made one using Electron, but then I checked Swift/OSX and ported there, loved the 8MB ram usage :D and settled there. But it's really easy to make one on Electron and have it cross-platform [1].
Did you ever release yours on the app store? I'm struggling with electron packaging for the Apple app store and hoping to find a kindred soul to help get through it!
I would love to see something as well-packaged, but for Linux. Most of the answers that people give end up being QT APIs or other dev tools. Bitbar is just so much easier... Put a shell script in a directory, and done. I would pay money for this in Ubuntu/Elementary OS.
Depends on the desktop environment. I use xfce4-genmon-plugin, close enough?