My guess would be the outside world knows about the Titans (iirc it was part of the since forgotten history that the reason other cultures ended up wherever the Walls are the was that wealthy fled on ships) was that whatever continent the Walls were discovered on (my guess would be Western Europe the exact size of all three Walls take up is as many small European countries) and that it was considered a dead, dangerous and forgotten part of the world and either whatever is behind the titans or the Wall Cult supported this in how they censored information and actively discouraged extended expeditions.

Remember that movie the Village? Yeah.

Neuromancer is p much the progenitor of modern science fiction and why things like the Matrix exist. Loooooot of cerebral bullshit and also I owe it another read through tbh. Great series. You really only need Neuromancer but its collectively known as the Sprawl trilogy. And also I'd love to do an AU of any flavor be it futuristic or something more modern I'm never one to turn down an idea. If you'd like my contact information I can be PM'd through this journal anytime!