Supposedly methods and sources. I imagine there may be some people still alive today who gave information and were promised confidentiality.
There certainly could be individuals who need to be protected. At some point, they will be gone and that will not be a valid reason. But "methods" could remain a valid reason indefinitely. In that case, I propose that they name an individual (such as Morley or whomever) to represent the conspiracy people. This person would work with the CIA and be allowed to see the files. They would, of course, have to sign an agreement not to disclose verbatim what is there under penalty of prosecution.
This person would be well-versed on the major issues surrounding Joannides, Phillips and Angleton and any other areas. They would study the files and prepare a report (that the CIA would clear) that could be released that would answer as many questions as would be allowable. I understand this is not a perfect solution, but it is better than nothing and may be the only way to partially resolve the issue. Just my 2 cents.