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Morrow systems ran on a Unix flavor, could address 16MB all on Z80, had memory protection and could have up to 6 simultaneous users. Micronix was the only Unix-like multiuser system ever to run on an 8 bit processor.

https://ia902902.us.archive.org/20/items/mame0.211manualsful...

Until Fuzix of course.

S100 systems were sometimes called mainframes after their card cage construction: usually with the qualifier “s-100 mainframe” for the system or the enclosure and backplane.

If it wasnt some mostly custom system it was probably an HP 64000 Logic Development System. Which used Z80s to run user facing software and control the actual mainframe. I doubt it was a pure Z80 "mainframe". Loved to be proved wrong though!