We tried 202 and 204 and both led some UAs to show broken image placeholders. But during the time we did that people assumed that we were tracking them just incompetently ("Look! They've revealed themselves!" style).
Maybe we tried some other codes but anything but 200 was unsafe to many UAs (you could 3xx but UAs would break on 304 too because the tracking pixel wasn't actually cached). Anything that led to UA breakage was verboten anyway on our side since we didn't want anyone to have a broken experience because they set DNT. That would have been bullshit.
We were dumb-enough to handle P3P headers too (which AFAIK no one really used in the end). Lots of dead code. Ugh.
Maybe we tried some other codes but anything but 200 was unsafe to many UAs (you could 3xx but UAs would break on 304 too because the tracking pixel wasn't actually cached). Anything that led to UA breakage was verboten anyway on our side since we didn't want anyone to have a broken experience because they set DNT. That would have been bullshit.
We were dumb-enough to handle P3P headers too (which AFAIK no one really used in the end). Lots of dead code. Ugh.