In the more distant past, when we didn't have a lot of "markup formats", "plain text format" just meant the opposite of "binary format", where "plain" was just

In the more distant past, when we didn't have a lot of "markup formats", "plain text format" just meant the opposite of "binary format", where "plain" was just emphasising its human readability. But I wrote my first BASIC program around the age of 7, in 1984 and the first and shortly after learnt Forth on a ZX Spectrum. So that's my perspective. We wrote programs, which controlled a Roland DG DXY-880. That also used a plain text programing language. I was not speaking a lot of English at that time, so mirror translating back from Hungarian, we said, "textual format" or "plain, textual format" ("síma, szöveges formátum"), to contrast its simplicity with binary formats. (I was born and raised in Szeged though, so that could be just our regional dialect. How about you?)