The old internet is fascinating to me. (Gee, a neocities user who thinks the old internet was better. Who could've guessed?) There are a lot of abandoned projects on the internet - Many older than me - Which have mostly fallen into varying degrees of inacessibility or at least inactivity. So here are some! Also, a lot (all?) of these are collaborative fiction projects, because what can I say, collaborative fiction projects are cool.
An sci-fi "online shared-world project" made in 2006, and became a lot less active around 2015. About sci-fi fans finding a magic substance called handwavium and doing a bunch of shit in space. Still recieves a post about every month-ish on the forums.and a couple times a year on the wiki. Plus, inexplicably hosted on the forums for a still-active fanfic called Drunkard's Walk, so relatively unlikely to go down as far as I'm concerned.
An scp-style anthology of strange objects, which to obtain you have to suffer through odd trials. The website (down since 2020 but still on archive.org) was made in 2007, but supposedly came from a forum which i cannot find, so likely existed earlier. The only remains of it outside of the internet archive is a fandom wiki.