Even if you’ve used the Mac for decades, I suspect you have never fully understood the Fn key. Not helping is the fact that Apple sometimes calls it the Function key, but all Mac keyboards already have 12 or more numbered F-for-Function keys! The Fn key first appeared in 1998 in the PowerBook G3 Series (Wallstreet) and has become a fixture in the lower-left corner of laptop keyboards ever since. The Fn key migrated to standalone keyboards only in 2007 with the release of the Apple Keyboard with Numeric Keypad, where it occupies a spot between the Delete key and the Home key. On Apple’s compact desktop keyboards, it reverts to the lower-left corner.↫ Adam Engst at TibBITS
This article made me wonder when the Fn key first appeared, but searching for it leads to a lot of SEO spam that is clearly all wrong. As the article notes, Apple first introduced it in 1998, and IBM already it in on the monochrome ThinkPad 300 in 1992. It’s much older than that, though; the IBM PC Convertible from 1986 also had one, as did the IBM PCjr from 1984. At this point I’m starting to think it was actually a fairly common key, but with the explosion of IBM compatibles in the early ’80s it’s basically impossible to check them all, and, of course, there’s the possibility it may have existed on earlier systems, or third-party keyboards long lost to time.
This would be an interesting mystery to solve.