The Iran Contra affair was investigated by special counsel (actually "Independent Counsel" as it was called back then) Lawrence Walsh.
I found his final report on archive.org but it's a bit too long for me to go through and figure out from the primary source if it has been released completely uncensored and when that happened. There might have been some material redacted/classified on national security grounds, at least back then. Was that actually the case though?
So, basically, what's the release history of the Walsh report?
The National Archives page on the records of the Walsh investigation has this header:
The records of Independent Counsel Walsh include security classified and otherwise sensitive information.
But that's for all the records of the investigation, not just the final report. Perhaps the report was drafted from the get go to exclude all such information...
I did find one book saying that:
Whenever Walsh's investigation produced results, it was stifled by claims of national security secrecy or classified matters. When the final report was finished, its Volume III was classified and never released.
But I'm not entirely sure of the reliability of this book (it's not an academic book). But it does offer a more concrete path to investigate (does this volume 3 exist, and was it classified?)
An academic source does confirm there was a classified vol. 3.
In addition to the unclassified Volumes I and II of this report, a brief classified report, Volume III, has been filed with the Special Division. The classified report contains references to material gathered in the investigation of Iran/contra that could not be declassified and could not be concealed by some substitute form of discussion.
Given the way this formulated, clearly the counsel segregated classified material to this volume. A comment below also says this volume was eventually released (to the public, I assume)--the question remains who decided its release. The full text of volume 3 is not available anywhere online, but a table of contents for it is. It starts with
Order granting a motion to release the Final Report of the Independent Counsel for Iran/contra Matters, December 3, 1993
So it sounds like there might have been some litigation surrounding the release of volume 3.