Howto Add Backports Repo to unattended-upgrades

You can check allowed repositories through debug argument of unattended-upgrades and then just uncomment backports line:

EDIT: also allow stable-updates or ${distro_codename}-updates for updates to clamav.

unattended-upgrades -d

Unattended-Upgrade::Origins-Pattern {
        // Codename based matching:
        // This will follow the migration of a release through different
        // archives (e.g. from testing to stable and later oldstable).
        // Software will be the latest available for the named release,
        // but the Debian release itself will not be automatically upgraded.
        "origin=Debian,codename=${distro_codename}-updates"; <-- uncomment this line
//      "origin=Debian,codename=${distro_codename}-proposed-updates";
        "origin=Debian,codename=${distro_codename},label=Debian";
        "origin=Debian,codename=${distro_codename}-security,label=Debian-Security";
        "origin=Debian,codename=${distro_codename},label=Debian Backports"; <-- uncomment this line

        // Archive or Suite based matching:
        // Note that this will silently match a different release after
        // migration to the specified archive (e.g. testing becomes the
        // new stable).
//      "o=Debian,a=stable";
//      "o=Debian,a=stable-updates";
//      "o=Debian,a=proposed-updates";
//      "o=Debian Backports,a=${distro_codename}-backports,l=Debian Backports";
};