@Rose_On_Mars z/OS I would say would definitely fit the bill, although arguably it didn’t “diverge” from office computing depending on what is meant by “office computing” (it was never intended to e.g. run personal desktop office software.) The *nixes would maybe sorta count, except that like tons of mobile devices run on what’s basically Linux and macOS is just FreeBSD with an Apple sticker. I mean I’m pretty sure there’s still stuff out there running on like VMS