The PDP-8 didn't really have undocumented instructions. It's just that the OPR instruction was "execute immediate as microcode word". There were a lot of ways you could flip those bits to get different effects, and people gave some of them their own mnemonics in third party assemblers, but the effects of the bit patterns were fully publicly documented by DEC and distributed with the machines.

Same was true with pretty much all non microcomputers through the 80s.