By backing up everything in important-looking folders (including My Documents, My Games, the Minecraft installation folder), and overlooking AppData as it is 99% download caches, tempfiles, and other garbage.

:) and local state, configuration, and anything an app needs when you do stuff. It's really the local user state in most cases, very important to back up!

On my Win10 desktop it got to 30GB though and the stuff worth backup is actually about 1GB of it.

Let's just call it what it is: a complete mess. Some apps install their binaries in there, some just config files, some all your data.

If there are guidelines (and they are not conflicting) about 1/3 of apps are ignoring them.

Also good luck trying to back it up with robocopy or anything, maybe windows' own backup tool can read it while running on that account, I usually get errors, which I think are mostly fine to ignore.

There's no good guideline - if you want a local cache that transports across user migration, that's the show. Windows doesn't really have a "separate your local disk cache from your backups" method, but you can certainly do the Cleanup Task to remove some things that buy into that modal that's 25 years old :)

I made a similar mistake when upgrading my SSD. There was some token that was stored in there that I needed, and IIRC I had lost the password needed to generate a new token. I only realized that I was even missing it a few weeks later when I needed to use the tool again. luckily I still had the old SSD so nothing happened. This happened even though I'm familiar with locallappdata and have even used it in some apps I created. It's just not the first thing you would think about when doing a backup unless you've already been bit by it before. Apps generally don't tell you if they store something important there, and the tool I was using never tell me where the token was stored. Doesn't help that file explorer hides the folder from you by default.