In cases where one wishes to negate a word whose "un" form is not standard, I prefer a hyphenated "non-" prefix, and would regard the usage as being the closest thing to a standard way of indicating the explicit opposite of a term. For example, an article about digital filters described the difference between linear and non-linear filters as being comparable to the difference between kangaroo biology and non-kangaroo biology. Even though the compound word "non-kangaroo" might never have been used anywhere previously, it clearly expresses its meaning, and a reader would be unlikely to worry about whether "non-kangaroo" was a "real word".
Additionally, in choosing between "non-redactable" and "unredactable", I would regard the former as clearly describing an inability to redact information, while the latter might be parsed as "unredact-able", meaning "able to unredact". The latter usage might make sense if one wanted a surveillance target to think he had destroyed information even though the information was, in fact, recoverable, but would not seem a good way to describe information which was conspicuously incapable of being redacted in the first place.
All that having been said, unless you are designing a versioning database which allows one to edit the "current" version of something but will maintain a journal of all edits, I think the term "append-only" might be more suitable.