For that matter, the "second" version of UNIX ran on a PDP-11/20 with no memory protection or MMU, and there were a few versions after intended to run on similar hardware (LSX, MINI-UNIX).

The PDP-11's MMU option was closer to the 8088's segmentation model I think, but I've never coded either, so dunno really. It does seem like it was possible to port "PDP-11 UNIX" to a lot more platforms than would get "VMUNIX".