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I do actually. My last job had a mainframe team maintaining (and adding to) an AS/400 application. They still had punchcard programs.

They had json apis. Each one had some variation on parsing http from a raw tcp connection with IBM RPG. I had to do some unspeakable things to a ruby library so I could control the order of the headers.

Looks like it was IBM System/360 mainframes but they've recently migrated to google hosted services.

actually, I do :) is it DOS? some IBM mainframe OS? do tell

Was DOS actually every used on mainframes/servers on a significant scale? (genuine question, not saying it wasn't)

A mainframe OS called DOS was in fact quite popular, but it’s not the same thing as the DOS that was in PCs. (There were others, too, like Apple ][ DOS. As soon as your computer gets the capability of attaching a disk drive, somebody has to write a Disk Operating System.)