Comment 1 for bug 1900185

This appears to have been lost as part of a refactor of the debian-cd branch. Looking at the branch history, this file was added in order to support something called Super GRUB Disk, which I've not heard of or used before. Is this why you're looking for loopback.cfg to be readded?

This seems to be a very narrow use case (why write an ISO as a file on the disk, instead of writing the ISO to the disk?), but also easy for us to fix.