Who remembers boundary areas and the kind of fence they have? Access to them is restricted and people don't usually have a reason to go there anyway.

After they were abandoned there's still no reason for them to enter cultural consciousness. They'd still be out of the way shrubberies on the back of fields, with no indication that they were part of the longest shrubbery in the world.

And even if they knew about that piece of trivia, were they expected to pass it along to future generations? (!)

Yeah, if you don't cross state boundaries (or even leave your village) like most people at the time the extent of the fence isn't really that important. Paris had substantial stone walls to enforce its own tax boundary at one point; since bits still exist plaques commemorate its existence, but I'd be surprised if the average Parisian thinks about the line of the wall much. The bit that got enshrined in the popular consciousness at the time is how much they resented the authorities levying the tax, and it appears that likewise the tax itself - not the implementation detail - was on Indian nationalists' grievances about British colonialism.

There still could be accounts in the native press though, but it looks like none exist. How come largest shrub wall is non-existent in Indian language press but existent in English press? No contemporary politician has referenced it in his speeches? No politician protested with British against this shrub wall?

Hm, I'm missing the point! Would anyone be so kind as to fill in with what's implied here? Thanks

There have been huge genocides in the 20th century which would have been forgotten if not for extensive retrospective documentation. Cultural memory is pretty selective.

Wonder why mamoths and dinosaurs had to be discovered

Hazy analogy but will do. Buddhism was made to go extinct in India similarly.