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.
I wish there was a modern version that isn't macOS.