Debian – unattended-upgrades does not upgrade packages from backport

I don't understand why you are doing here. Why do you have a preferences setting for stable at all if you are running a stable system? As far I know, no preferences setting is necessary for stable in that case.

You don't explicitly say whether you are running a stable system (you really should say so), but if you are not, then I really have no idea what you are doing.

And if the release is on stable, then the usual thing to do for testing and unstable is to set their preferences to less than 100. I usually use 50.

And if you want to downgrade to stable, just do the following (assuming sane settings like the ones above) to downgrade pkgname1 and pkgname2:

apt-get install pkgname1/stable pkgname2/stable

This sets the specified packages to the target release stable.

Incidentally, mixing testing and/or unstable packages with an unstable system is generally a bad idea unless you know what you are doing. Some of the time it is Ok, but most of the time you need to use backports, either from Debian, or self-made.

systemctl mask unattended-upgrades

Explanation:

systemd units can be overridden by the adminstrator putting a file with the same name in /etc/systemd/system.

This mechanism can also be used to "mask" a service from being activated by socket activation, manual starts, or any other method. Instead of creating a file with the same name, if there is a symbolic link to /dev/null, then the unit is effectively ignored.

So you can ab(use) systemctl mask, to replace the contents of the unit with nothing.

To avoid the possibility confusion in future, check that you remove the mask once you have removed the package. systemctl unmask unattended-upgrades.