In some redacted documents, there is even an alphabetical word index at the end with a list of pages on which the words appear.

The redacted words are also redacted in the word index, but the alphabetically preceding and succeeding words are visible, as is the number of index lines taken up by the redacted word's entry, which correlates with the number of appearances of that word.

This seems like rather useful information to constrain a search by such a tool.