What would it take for someone to make an exploration addon for Classic WoW...?
...showing all the discoverable areas of each zone, on the zone map?
I'm guessing this isn't even possible to do by digging into the code, as each boundary is drawn I-don't-know-how on the actual world map?
I'd settle for an addon that just showed a list of each explorable location per zone on the zone map, and ticked them off automatically as you discovered them.
There would've been a handynotes extension made if there was something like achievements like there is in retail but, with nothing gained from accomplishing what you're asking makes this a very niche addon. It would more than likely take someone that codes and is interested in doing the same as you.