Amusingly enough, people have even screwed this up. I recall some government agency trying to censor data with a black bar, but the problem was that the data was in a SVG-like document, so people could just delete the bars from the document and see the apparently-censored text.
Yep - Apple wound up adding a “Redact” tool to Preview.app for PDFs, likely in response to all the people who just drew shapes over selectable sensitive text.
I believe the US government did this at least once, by implementing the black bars in a PDF as a separate object, so the unredacted item was intact in the files they shared.
