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.
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.