Why not twm? It's got some deep roots, being around almost 30 years. I think
only uwm might be older. (uwm was "the only X11-compatible window manager as
of X11R1" and "in 1986, the X Window System switched to version 11 of the
protocol. Only uwm was ported, so it became the only window manager for X
Window System until X11R4 release, where it was replaced by twm. uwm has
never been updated since.")
At least twm is still maintained! :)
https://en.wikipedia.org/wiki/Ultrix_Window_Manager