You are right, copying to dom0 is not best practices and warned against since anno dazumal, but given the user groups I remember not always being technically minded (journalists, dissidents, etc.) and ensuring qubeses isolation holds even when users do things they are discouraged from has always been part of the philosophy. Don’t trust users, don’t trust userland, don’t trust software and all that yazz.
I believe this is a vulnerability that occurs when copying data from dom0, which is a more common task. Generally the Qubes model recommends copying data from more trusted VMs to less trusted VMs, and dom0 still runs some system-wide processes in many default configurations.
For example when you take a screenshot with xfce4-screenshooter, the file is saved to dom0, and you have to use qvm-copy-to-vm to move it to a (less trusted) qube to do something with it. That's the most frequent use case, at least for me.
