You can find it backed up on the Wayback Machine: https://web.archive.org/web/20180105104931/https://www.lesswrong.com/tag/conspiracy_world Additionally, links to the same content are available on the LessWrong wiki at https://wiki.lesswrong.com/wiki/Beisutsukai.
Edit: Additionally, to address your first question, tags were likely lost in the shift to LessWrong 2.0, but I haven't been around long enough to confirm that.
Yeah, tag-links were quite rare, so we didn't port them over for dev-time reasons. If lots of people want them back, we can probably figure out some way to make them work again.
For any collection of posts that people want to link to, I recommend creating a sequence on the Library page. Anyone can create any sequence there, and a bunch of other users have already ported over most of the sequences listed on the LessWrong wiki.
Can one of you tell me what happened to /tag/conspiracy_world ? I could not find it anywhere I looked online, as the only links lead to that page, which says that it couldn't find the content.
I came to search it because of Eliezers "Final Words", 27th April, 2009. I would like to read the rest of it.