It would probably make sense to also include the other standard highlight arguments like stroke, etc. For example, a draft document could mark proposed redactions like this:

Isn't something like this already doing the job?

// simple redaction
#let redact(body, color: black) = box(hide(body), fill: color)
// redaction as in the issue
#let redactre(body, color: black) = {
  show regex("[^\s]"): it => redact(it, color: color)
  body
}

Isn't something like this already doing the job?

// simple redaction
#let redact(body, color: black) = box(hide(body), fill: color)
// redaction as in the issue
#let redactre(body, color: black) = {
  show regex("[^\s]"): it => redact(it, color: color)
  body
}

That does not hide the size of redacted words

Then I do not understand what the intent is. By using "." as the regex expression, however, everything would be marked.

The regex approach still leaks information in the PDF output because the boxes contain information about the size of the glyphs or words. See https://www.wired.com/story/redact-pdf-online-privacy/

“Even if you do the redaction, supposedly correctly, even if you remove the text, there’s a lot of latent information that is dependent on the content that was redacted, and even that can leak information,” Levchenko says. “If you redact a name in a PDF, if the attacker has any context—they know this is an American—they will be able to, with high probability, either recover that name or narrow it down to a very small list of candidates.”

Edact-Ray focuses on the size of glyphs (broadly, characters or letters) and their positioning. “It’s pretty clear to a lot of people that the letter ‘L’ is skinnier than a letter ‘M,’ and that if you redacted just the letter ‘L,’ then you might be able to tell it is different from a redaction with just the letter ‘M,’” Bland says. The tool is essentially able to automatically compare the size of the redaction and the position of the letters with a predefined “dictionary” of words to estimate what has been replaced.

The code below shows the several issues. The first paragraph shows how #hide properly spaces, but does not communicate that there was a redaction. The second shows #box breaks the line if the selected overlaps into the next line. The third paragraph shows how #regex leaks the size of the glyphs in letters or words into the PDF file (even if it appears as all black that runs together).

// simple redaction
#let redact(body, color: black) = box(hide(body), fill: white, stroke: red)

// redaction as in the issue
#let redactre(body, color: black) = {
  show regex("."): it => redact(it, color: color)
  body
}

#box(stroke: black, inset: 1em)[#lorem(10) #lorem(10) #lorem(5)]

`#hide`
#box(stroke: black, inset: 1em)[#lorem(10) #hide(lorem(10)) #lorem(5)]

`#box`
#box(stroke: black, inset: 1em)[#lorem(10) #redact(lorem(10)) #lorem(5)]

`#regex`
#box(stroke: black, inset: 1em)[#lorem(10) #redactre(lorem(10)) #lorem(5)]

Well I guess one could create a broken/bad implementation by start & end point, text height, content size and line spacing measurements, but a good solution seems not possible to me with the current typst version.

How about something like this?
The function redact measures the width of the text that is to be redacted (say, 3.213cm), then it rounds that width to an integer millimetre (32mm) and creates that many 1mm-wide black boxes to fill the space.

• The output pdf should not contain the text (though I don't know enough about Typst's internals to guarantee that).
• You have a black bar that breaks at lines, but only tells an adversary roughly how wide the redacted text was (no information about individual glyphs).

I also tried to measure how much space is left on the current line (with layout, here().position(), etc.) but didn't succeed. If this was known, we could output one appropriately-sized black box per line (instead of one box per millimetre).

#set page(paper: "a6")

#let redact-box(width) = {
  box(rect(height: 1em, width: width, fill: black, stroke: none))
}

#let redact(size: none, body) = context {
  if size != none {
    redact-box(size)
  } else {
    let redact_size = measure(body) // size of the text that is to be redacted
    let redacted = int(redact_size.width.mm()) // width in number of mm's
    // create a 1mm-box for each mm
    for i in range(redacted) {
      redact-box(1mm)
    }
  }
}

Pursuant to Federal Circuit Rule 47.4, counsel for Petitioner certifies
the following:
1. The full name of the party represented by me: #redact[Public Co.], Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim aeque doleamus animo, cum corpore #redact(size: 2cm)[dolemus], fieri tamen permagna accessio potest, si aliquod aeternum et infinitum impendere malum nobis opinemur.
2. Quod idem licet transferre in voluptatem, #redact[ut postea variari voluptas distinguique possit, augeri amplificarique non possit. At etiam Athenis, ut e patre audiebam] facete et urbane Stoicos irridente, statua est in quo a nobis philosophia defensa et collaudata est, cum id, quod maxime placeat, facere possimus, omnis voluptas assumenda est, omnis dolor repellendus. Temporibus autem quibusdam et.