Here's a fun exercise: pick a random old post, ideally one distinctive enough that you won't find much similar elsewhere, and google it. Then, go to the very last few pages of results. If you picked your phrase well, you will find a bunch of automatically-generated websites with random urls and content composed of low-res memes from decades ago, what might be CP, and pages upon pages of mangled stolen posts from various boards.
Maybe it's some elaborate way of getting money from ad revenue, but I don't understand how. No one will be visiting a url that looks like it came out of Satan's anus. There's definitely some elaborate money-making scheme tied up in there somewhere.

As for the spam that imageboards/textboards get, is it possible that the spambots used also operate on other types of boards as well? Maybe some relatively common spambot software package comes with built in support for various bits of imageboard software. I'd imagine that the spambot's authors implemented support for spamming Wakaba and Kareha boards back when there were far more around. Interested spammers continue to pay for use of said spambot services and they coincidentally end up including imageboards as a target. I expect that good spambot software is intelligent enough these days to automatically detect what is a forum it can post on.

I've always suspected that the CP ad spam could be federal agents trying to bait paedophiles into revealing themselves. That, or some pissed-off imageboard admins trying to get competitors shut down. Or maybe the links go to malware downloads that some skiddie is using to build up his cryptocurrency mining botnet. Probably all three, really.