OpenBSD/pmax used to run on a large set of the Digital MIPS-based DECstation and DECserver family. These machines were the counterpart of the VAX-based VAXstations, until the Alpha line was ready.
The OpenBSD/pmax port was discontinued after the 2.7 release.
The source code is still available in the CVS Attic, and code can be found in NetBSD as well.
The pmax port of OpenBSD was started by Thomas Graichen shortly before the OpenBSD 2.0 release, based on the NetBSD/pmax sources. It was then slowly brought in shape, becoming self-hosted, and moving to ECOFF, then ELF binaries.
A few releases later, Thomas Graichen stopped working on this port, and nobody stepped in. The port was slowly left in oblivion, with no new code and hardly any bugfixes, although releases were still being made.
A timid effort was made after the 2.7 release to revive the port, but MIPS-specific toolchain problems prevented the system from building itself for a long time, and development went on hiatus again. When the problem was finally fixed, it was decided however to stop supporting it and remove the code from the tree.
The last supported OpenBSD/pmax release was OpenBSD 2.7. It is not available on ftp sites anymore, but releases up to 2.3 were available on CD.