[Rochus]

Yes, I had some projects in the late eighties/early nineties, with expert system shells like M4 by TechKnowledge, CLIPS by NASA and some homegrown shells based on Lisp or Objective-C. Not that scary at all, but the main challenge was the rules (where to get them from, what to do with incomplete or conflicting rules, etc.). CLIPS is still available (see https://www.clipsrules.net/) and very well documented.

[ZevsVultAveHera]

Cool. You get some fancy editors for input or was it some text files or other format?

[Rochus]

There were not many fancy editors at that time compared to today's measures. As far as I remember, M4 came with its own (not that fancy) development environment. For CLIPS there was the REPL and some primitive IDE.