I know that in the 2000s a number of popular pieces of software were written in C# for Linux. To list a few:
There was also a IDE called Monodevelop. Is it still possible to develop C#, GUI software for Linux? Or is the whole idea of Mono dead?
Monodevelop is still actively developed
It's no longer packaged for recent versions of Ubuntu
You will have to setup their own repo
Though less popular than it used to be IMHO for desktops, Mono itself is still actively developed and recently mono on mobile platforms.
