Yes, it's been that way long before PDFs. Simply knowing the potential words, often names, that could appear in a document, gives those with the redacted documents a chance at determining what has been hidden based on size. This might be part of the reason why when declassifying documents, the redactions end up being more of a sentence than is needed. The extra buffer of hidden words gives some additional protection to what needs to be redacted.
This reminds me of one of my proudest moments in high school.
For a test in German class (my worst class), the teacher had just used tippex to remove some words and put them next to the text, and we had to fill them back in. I grabbed my ruler and measured all the sizes. There was 1 very long word, many medium sizes and a few smaller ones, but with this information and the context of the text for the first and last time I was able to get my first and last 10/10 in this class.
A malicious "redacting" algorithm submitted to the underhanded C contest used a similar idea, just on lower level.
PNG allows ASCII numbers, so flipping all digits to 0 creates a pixel which is graphically "masked" but leaks information about the original pixel: "000" means the value was larger than 99.
