This is a supplement to the Archive Team's Deathwatch and Fire Drills. Most of these are just ideas I had jumping around in my head, and they might be subject to changes.
dedicated handhelds
Discord logs with DiscordChatExporter
DLsite was blacklisted by several payment processors and some of its games have been deleted. Somewhat justified for material considered illegal overseas, though the SFW sections (home, soft, comic, app) probably should be mirrored.
GBATemp's Wiki just refuses to be scraped!
Internet Archive because INTERNETARCHIVE.BAK is paused at the moment.
itch.io is a mixed bag, though some of its better games aren't mirrored elsewhere in case it goes down again.
Laserdisc/LaserActive rips
LiveJournal
Neocities is fine at the moment, though it could become a good source of modern history.
new arcade games
obscure Fandom wikis
Online-only games
OpenLeaks and WikiLeaks
osu! is being overhauled. Original charts partially cloned in osu!octave's data
Paramount+ delisted The Fairly OddParents: Fairly Odder. That show is probably not a big loss, though should we download other streaming exclusives just in case?
remywiki
The SCP Foundation is still being cloned.
SocietyGlitch is a private torrent tracker dedicated to rare documents, photos, and videos. While some of its media have been mirrored on Internet Archive and reviewed on Oddity Archive, others could become even harder to find if the primary source shut down.
Touhou Wiki
The first Xbox's DLC has become scarce after the closure of its Xbox Live.
Android apps/games
FDS QD files
N-Gage games
VMU communications are not properly emulated yet, meaning that the Chao Adventure games can only send powered-up Chao to the Sonic Adventure games on real hardware.