Well, by "relatively popular" I meant that AS/400 servers (aka AS400 or IBM iSeries or IBM System i: https://en.wikipedia.org/wiki/IBM_System_i) have been quite popular mostly only within certain industries and/or niches (e.g., banking, insurance, manufacturing, supply chain). They are still widely used there for legacy reasons, however demand for AS/400 developers (which implies proficiency in RPG language, since, as I understand, most legacy applications are written in RPG: https://en.wikipedia.org/wiki/IBM_RPG) is tiny; so, it's a good language to know to potentially expect a relatively high salary or consulting rate (hello, Delphi!), but a dead end for IT career growth. I would also venture to guess that some (many? most?) companies currently using AS/400 plan to migrate their legacy applications to the cloud.

"Popular" was a touch sarcastic. AS/400s were most recently called the IBM system I, and are now a discontinued product. IBM still sells mainframes, and they have discontinued the AS/400.