> That's a very recent feature, isn't it?

Android 12 theoretically added it but it didn't fully work yet and apps needed to start targeting Android 12+. Then, the backup app needed to add it which took a long time, so it's fairly recent as in within the past 2 years.

> Curious how many apps abuse that for purposes beyond the original intention? The big/popular apps etc? Ie what will be the real world experience despite this new feature

Many apps use the older system to disable backups or exclude files which only applies to cloud backups now. Most apps don't exclude files from device-to-device backups improperly. Excluding device-specific login tokens, caches, etc. is how it's intended to be used. It annoys people they need to login to apps again after a transfer but that's generally how those apps want their security model to work so there's an entry for each login session and logging out of it only logs it out on 1 device.

Apps can exclude files and define their own backup agent for handling converting to and from a portable format. Chrome does this to backup Android-specific settings, etc. not handled by Chrome's Sync feature. This means non-Chrome Chromium-based browsers largely lacking sync have poor backup support. We plan to address this in Vanadium but haven't decided how we want to approach it yet.

It does work well. People mostly have a good experience with device-to-device transfer with Google Mobile Services devices now. Certain apps like Signal or TOTP apps using the hardware keystore inherently can't be backed up this way. Signal doesn't use it in a sensible way for security and really just seems to have introduced hardware keystore based encryption to prevent using root-based backup systems not taking into account which data should be backed up or not backed up. They don't trust the OS backup infrastructure particularly since OEMs can include sketchy backup services so they don't want that to work either.

Thanks for the comprehensive reply! I gave it a go today with a USB stick. Just have to test a restore now :)

Backups are per-profile so you can test restoring most data in a temporary secondary user, just not settings it will only restore when restored in Owner.