OpenBSD/arc used to run on the machines compatible with the Advanced RISC Computing specification, known as ARC machines, based on MIPS processors and initially designed to run Microsoft Windows NT. Such machines included the long dead Acer PICA, as well other machines manufactured by other companies such as MIPS, Deskstation, NEC, and Olivetti.
The ARC specification is extinct, and no new ARC BIOS machines for MIPS are likely to ever be manufactured. Microsoft has stopped supporting MIPS platforms after NT 4.0. OpenBSD/arc used to provide a good alternative to NT!
The OpenBSD/arc port was discontinued after the 2.3 release.
The early history of this port is not very clear. Apparently the first work was done by CMU as part of their Mach project. The initial hardware was the DEC R2000/R3000-based DECstations. This code was later used by both the Sprite and BSD groups. The 4.4BSD code, known as the pmax port, was made freely available in mid 1993. It was merged into the NetBSD tree by a variety of people, but took several years to really become stable and mature, mainly because of compiler toolchain problems.
Per Fogelström became familiar with the code after porting it to a home-built IDT R3081 based board. Subsequently he added R4400 support when porting it to the MIPS R4400 Acer PICA board. Willowglen Singapore purchased a second PICA board for Theo de Raadt so that he could improve the port for use as a development system for an internal project. Since then Theo, Per and others have completed the port.
As a result, the code has been modified to make it more versatile, and eventually support a larger range of ARC machines. Unfortunately the death of this platform, as well as the lack of general availability of this hardware, eventually turned people away from working on this port. Eventually, it was decided to stop supporting it and remove the code from the tree.