I think it traces back to the early WEG days. WEG provided the very backbone, the foundation, for everything we know about the Star Wars universe outside of the films.
They did a lot of awesome work expanding the universe BUT there was one aspect of it that was eventually abandoned but the echoes of which could still be felt going forward. That aspect being that the original contention was that most of the galaxy was still unexplored and it was new founded Empire that was leading this charge to explore what was out there and to conquer, exploit and enslave anything and everyone they came across.
For the longest time, it was a fact that Mon Cala/Dac was a newly discovered planet within the OT timeframe.
So with that context in mind, the Unknown Regions being laid out as they were made some sense. But it should have been readdressed loooooooong ago. That they’ve not done so at this opportune moment in time with the new canon is a massive failure on their part.
Well, hasn’t the EU more or less always implied this? Even in the early Marvel comics they talk about how each different species/culture have different starmaps. Han Solo talk about buying starmaps from aliens in order to find unknown/unexplored planets to hide his cargo on. It’s implied that these planets are hiding in-between the well-populated sectors (Hutt space, Corporate sector, etc.)
I also seem to recall from the WEG Sourcebook for Tatooine that they referenced how the planet had been discovered and it’s location added to the Republic’s starmap database, but since it was just an empty desert world of little use it remained unsettled for several thousand years. By the time of Luke’s birth there had only been humans there for a few centuries (roughly, I can’t remember the exact numbers).
So it seems that the EU has always, although to various degrees, been not to far away from what been suggested on this thread so far. That the Core world have always been aware of what’s around them and where they can go, but for economic reasons, not to mention safety, they’ve only explored so many places.
One could also argue that planets like Hoth and Endor were undiscovered until the Rebels and Empire found them. And they’re a fair distance from the Unknown Regions. And according to the WEG EU, there were planets in Hutt Space (e.g well explored regions) that were ignored for thousand of years due to economics.
PS. In which WEG book was it established that the Mon Calamari were recently discovered in the OT? Was it a Sourcebook, or the book called “Death in the Undercity”? (I have a copy that book, but I haven’t looked through it yet.)