>>1626316
>what measures can we think to archive everything about Touhou?
Buy hard drives, download everything yourself. Make redundant copies. As shown by the 2019 sadpanda incident and the 2022 DMCA incident, mass mobilization doesn't happen until AFTER shit starts getting taken down. Preparing earlier will give you a big headstart. When the archival effort actually starts, you'll be able to help fill the gaps and replace what's lost instead of chaotically scrambling to download what's still available. And if there's no mass effort, then at least (You) will have it all and be able to share with whoever needs it.
>What are the most important things that should be archived?
My personal opinion, from least to most concern:
<official games and music
Incredibly unlikely to ever be lost, you should already have them for personal use. Consider grabbing the old static patches/standalone thcrap patches if you want to be extra safe. Make sure you have vpatch, ultra mode patches, adonis2, soku/giuroll and other utilities.
<fanmade music
If it's in the TLMC, it's not going anywhere. Consider downloading for personal use if you have 5TB to spare.
<official manga and printworks
Unlikely to be lost, worth downloading for personal use. Remember to grab the small stuff like printwork extras and ZUN-approved doujin manga.
<fanmade anime
Please download and seed the Touhou M-1 Grand Prix. You're unlikely to find it all in one spot after the seeders dry up and also it's very very good.
https://nyaa.si/view/1721481
The other three big anime projects are rather famous so they'll 
probably be fine. Anything less known should be downloaded on principle. Things like Burning Meiling, kawayabug's Owl and Bat, don't expect to find them again if they're taken down. 
Remember to save the subtitles too.
<fanart
Less vulnerable to censorship and more to their own creators. If you like an artist, grab their work before they delete it. Your two best friends are:
https://github.com/mikf/gallery-dl
https://github.com/Nandaka/PixivUtil2
<fanmade doujins
Download absolutely everything you like, H or not. Neat software I found recently lets you download Danbooru pools with translations intact:
https://github.com/nostrenz/kosuzu
<fanmade games
In dire need of better archiving effort, most of the work done during the 2022 DMCA incident seems to have gone to waste.
https://nyaa.si/user/john_money_anon?f=0&c=6_0&q=&s=seeders&o=desc
The torrents made back then are 90% dead. Just like then, the best starting point for hoarding fan games today seems to be this torrent from 2018:
https://nyaa.si/view/1058859
Any old games not in there are download on sight. Newer games are a pain in the ass with all the DLC and updates to keep track of, archiving them is more effort than I'm willing to make. It's necessary work, though.
<fanmade everything else (music videos, gachi/cookie videos, literature, mods for other games, cursors, icons, desktop pets)
Download on sight, keep safe forever. It's important cultural heritage.
>What are YOU doing about this?
All of the above. New fanart gets scraped every morning, other content gets downloaded as I watch/read/play it. It's nowhere near a complete archive, but it keeps the stuff I care about safe with minimum effort. The stuff you care about, you should make an effort to keep safe. For yourself, for everyone, for future generations.