I'm not too sure what you're talking about. There's really no Neopian Land you can't access that has been "forgotten" (though there is one which everyone pretends doesn't exist). We know of some planned lands or in one case a land that is no longer accessible:
Jelly World: Starting off with the land everyone pretends doesn't exist, Jelly World. A world made of jelly. It's meant to be a joke world, the joke being that it exists yet its treated like it doesn't. They are one or two characters on the site that hints toward Jelly World but they're treated like their insane (one of which is insane). If you want to find out more about Jelly World you can check
this article about it on Jellyneo, a Neopets help site.
Lutari Island: A land you can no longer access, sorta. There's a place for it on the explore map you can click on but it takes you to a page where you barely see the land and you're told there's a terrible storm around it thus you can't get to it. Lutari Island used to be accessible not through the site but rather through a mobile service Neopets had,
Neopets Mobile. However when Neopets Mobile was discontinued so was access to Lutari Island. Since then no one knows what TNT (The Neopets Team, the staff who runs the site) plans on doing with it (or rather planned, recently the site switched ownership and staff so if the original staff did have any plans for Lutari Island who knows if the new staff knows or will do anything with it). To find out more about Lutari Island, once again, Jellyneo has written a guide for it
here.
Scurvy Island: There are a few "abandoned lands", lands which TNT thought about adding but it never came to be. One of these lands is Scurvy Island. All Neopet lands have a theme to them, take Krawk Island which is pirate themed. Occasionally Neopets had big event called "plots" where a story took place someplace on the site and user participated, usually either solving puzzles for battling. Krawk Island (and an aquatic/underwater themed land called Maraqua) had a plot called "Curse of Maraqua". One location during this plot was called "Scurvy Island" which (the original) TNT said was meant to be a more darker, grittier pirate themed island compared to Krawk Island's more kid-friendly, cartoonish pirates. However in the end TNT felt there wasn't that much of a difference between the two as they initially thought and scrapped it... sorta. Another event took place on Krawk Island that completely
redid Krawk Island's map and this new map included a far off island (its the one which has the "Keep Out" sign on it) which looks similar to what
Scurvy Island looked like. But like with Lutari Island after Neopets Mobile, though they added it to the map who knows what TNT plans on doing with it (or if the new staff even knows or will go with the plans if they do).
Bogshot Village: A planned swamp land, for whatever reason it just wasn't made. A location in the Neopet's PS2 game "Neopets: The Darkest Faerie", Bogshot village is a village that's in a middle of a swamp land somewhere between Meridell, a medieval land, and the Haunted Woods, a woodland that you can described as being Halloween themed. The main attraction of Bogshot is Sophie the Swamp Witch, who would later star as a main character in a Haunted Woods plot called "Tale of Woe" (and she may have been moved out of Bogshot into a secluded part of the Haunted Woods to do this, though you could say she's just on the border of the Haunted Woods and Bogshot). Really there's nothing else to it then that and a
TCG image of it, plus its role in the PS2 game.
Those are the main ones I can think of, though there's also
The Lost Isle,
Petaria,
Petpet Park, and
Portal City but those are special cases. Also on the
Explore Map you can see potential places for new lands if they ever decide to add a new one (though it has been mentioned that the southern pole of Neopia is mostly an icy wasteland).
