One interesting example is Josephus. A Jewish-Roman historian, he provides almost the only roughly contemporary mention of Jesus outside of biblical ones (and biblical apocrypha). He simply describes the existence of a 'superstition' (non-officially accepted religion) around Jesus, that he was crucified, and that his followers still venerated him.
Sometime in the last 2000 years, probably the Early Middle Ages, Christian scholars doctored this passage to have Josephus suggest that Jesus was a god, or at least superhuman (even though Josephus was not a Christian believer).
Modern scholarship, including religious scholars, almost unanimously accept the passage as fake. But Christian proselytizers still use it very frequently as extra-biblical confirmation of the divinity of Christ.
Please provide an example of an culturally important large-scale work of history or literature, at least 1,000 years old, which hasn't been abridged, extracted, expurgated, euphemized, hidden, bowdlerized, etc.
Of course now that the colonisers have reached the lands the sites are under threat from gas plants and industrial development .. but several thousand years was a good run.
I don't think that is true. There is lots of rock art which either can't be shown to outsiders (or to members of certain genders or moieties), or can be shown, but the significance behind the art is not allowed to be explained to outsiders. Some rock art can be copied for wider consumption, but the copies have to leave out certain details.
I don't think it is true that this rock art is an example of significant culture that has never been censored or expurgated. The taboos and rules which apply to viewing this art and/or reproducing it or the linked stories are surely analogous to a form of censorship. The fact that people have broken those rules does not change this.
It's true that various sites are considered the stories of particular groups ( men V. women, these people V those people) .. but aside from that specific consideration they haven't been censored in the sense of ( painted over | black barred | altered ) or hidden (cave | over hang moved or draped with a false rock).
The major thrust of this sub thread has been about line of original purity more than an obligation for [some group] to openly share to all regardless so forgive me if I still consider these a good example of unaltered works painted on and carved into stone.
I think you are splitting hairs. Control of certain forms of art or literature so that they can't be revealed to all is what we're talking about. The culture in question is very different from ours so we can't expect their taboos or censorship to have exactly the same forms that we might associate with censorship (mean-looking guy with eyeshade and red pencil, hooded monks chained to desks in candlelight, omnipresent super-computer says no, etc).
For example, some stories are not just 'the stories of particular groups' but are not allowed to be told by, or in some cases told to, other groups.
Besides this, how do you know that none of these works have been painted over or hidden?
For most works 1000 years or older, we don't know the full textual provenance, so providing an example in either direction is essentially impossible. But if the phenomenon is as widespread as you claim, it should be easy to find much more recent examples.
You are extremely naive. This has happened lots and lots of times.