I want to add a couple of good ones I’ve found:
Jeff the Killer lost media: no one knows where the original Jeff The Killer image came from.
Mortis.com: old weird website
Also here’s a good website on various obscure computer/Internet related oddities: https://suricrasia.online/iceberg/
There’s all the young kids who went viral in the early 2000s, often because the internet made fun of them.
The mystery was would they turn out to have horrible lives.
But it’s been so long they’ve all grown up and no one remembers them and they have normal lives.