It’s not like these instruction were ‘secret instructions’ reserved for use by a secret cabal. They’re just leftovers from ‘don’t care’ parts of the microcode. I remember there’s quite a few that simply hang the processor because their microcode doesn’t include the ‘increase the program counter’ instruction so it turns into an infinite loop.
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.
The original 6502 had many undocumented instructions. Most of them not very interesting, but certainly undocumented. http://nesdev.com/undocumented_opcodes.txt
Here is a detailed examination of undocumented Z80 behavior. http://www.z80.info/zip/z80-documented.pdf
The 8080 had undocumented instructions too.
So now we are all the way back to pdp series. The pdp-8 had plenty of undocumented instructions.
Should we go back further? It doesn't stop.