Well I know for 4chan, each thread is ephemeral and depending on how active a board is, the thread will be deleted. Which I think is the challenge citing it. Since even a day or two after finding it, it can be lost.
> "And you have things like the internet archive or Webcite[0] to make sure the web document doesn't disappear."
And then you just webarchive archive the archive sites instead of 4chan directly :-)