The rediscovery and investigation of the Plexus P/20 has been an amazing project in the retro community. Its great to hear that Adrian Black is offering up his restored version up to the Interim Computer Museum to make it available for people to get their hands on.
I watched Adrian's Digital Basement series on YouTube about this particular system. It was one of my favorite of his obscure hardware videos. Very cool!
I've been watching those videos after seeing this, it's interesting to see such an obscure system these days, most of the hardware from that era is usually well documented but I've never heard of this system before now
I wonder what the z8000 and vax commands do. I can imagine the first is related to the 16bit Zilog processor and the second to the vax OS, but no idea how to use them or what are they for.
As best as I can tell, they just return false, as in you aren't running on those processors. m68 returns true. All just hardcoded scripts, though, nothing fancy.