You wanted a source, I gave you a source.
Refer to the 2010 statement from the then creative director of WoW. Then refer to BFA where Goblins return to Kezan and indicate that Undermine is still there.As for "Maps" and ingame stuff. You realise that during development things change right? That doesn't mean it's a real location.
Uh, that's always been how Undermine has been portrayed. It's a hallmark of the Goblin race, and it's a hallmark of the lore behind the location. It's silly to blatantly ignore those facts simply to be argumentative.And again, we have no idea what the status of Undermine is. So claiming it can only be a city full of people who will accept the Horde/Alliance with open arms is just stupid. It can be whatever blizzard wants it to be.
Uh you're the one who brought this up in the first place.But enough of you dragging the thread off topic with more headcanon (Much like Tinker was). Until we know what the expansion contains, Undermine is as much as fairytale addition as Avaloren, China, Russia or Belgium.