Comment by gruez

>Of course many app developers decided the Google cloud is too insecure, being not end-to-end encrypted

But so far as I can tell D2D transfers don't hit the cloud?

>For a D2D transfer, the Backup Manager Service queries your app for backup data and passes it directly to the Backup Manager Service on the new device, which loads it in to your app.

https://developer.android.com/identity/data/testingbackup

If your app is opting out of backup by implementing a custom backup agent that returns no data, it's pretty clear you're against user backups, period.