They can probably tell the Culture about the time before the Warp became malignant.Do the Necrons have any worthy data on Chaos besides their warp null fields?
No.Do the Orks have any data on Chaos worth anything at all?
They have an instinctive dislike of it, and will ostracize or kill any Orks tainted by it. But that level of "knowledge" is far below what the Culture already knows.
Individual Big Meks or Weirdboyz might have some useful warp knowledge though.