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/
Got wrapped up into that a while back, an interesting tangent is the song So Do I by DCO, which was similarly lost until one of the band’s original members recognized it and posted it online. Said member did confirm that TMMS/Like The Wind is not a song they made, tho.