Well it depends on what you consider modern but Openbox is still alive and kicking (albeit it is only a window manager).

On a rant tangental note a slight annoyance I have lately is that all the window managers are getting pushed out / ignored because of Wayland.

With Wayland you basically have to write an entire desktop instead of just a window manager. I used to love that about Linux.. the pick and choose what works best model but now the choices are becoming fewer.

Oh well back to my macbook with zero choice.

The only wm I loved and stuck with on Linux was Windowmaker. Your comment is why - NeXTStep was awesome and missed..