If I recall, YouTube had a problem whee if you typed a period or something all the results would be child pornography or something. The videos didn't have any tags thag identified them as such, but you could access them by knowing the terms used.
I'm not sure you're remembering that situation quite right: I think what you're remembering is a situation on youtube where pedos were using naming patterns for default file names that devices produce to find obscure videos uploaded
by kids of them doing not sexual things like silly dances or whatever, probably for their friends, but not understanding that others might find them or why that's an issue. Pedos would "tag" and timestamp unintentionally revealing parts of the videos in the comments. There was a "rabbit hole" you could fall down with the algorithm where if you found one of those videos and watched any of it, you'd suddenly get tons of those videos recommended. I would
guess because the algorithm noticed over time that the people viewing those videos ONLY viewed those videos (pedos never change) and using that behavior as a template for anyone else that found the videos, suddenly your recommended would be full of grainy videos of kids doing kid stuff and pedos slobbering over anything they could find in the comments.
Your original description does remind me of what I vaguely remember hearing a couple years ago about how that shit got (and probably still gets) trafficked on twitter (though my google-fu fails me on that one): searchable gobbledegook terms in tweets with links to filedump services, that only meant anything to people "in the know". It's not always gobbledegook though,
the recent shit about what was happening on instagram involved tags that were extremely obvious.
