How do I disable fetching of DEP-11 files

If you dont need graphical package manager you can uninstall appstream package and all graphical managers that depend on it:

aptitude purge appstream

If you prefer mv solution, then better do it with dpkg-divert and it will stay persistent on package upgrades:

dpkg-divert --local --rename --divert '/etc/apt/apt.conf.d/#50appstream' /etc/apt/apt.conf.d/50appstream

Looks like it is defined in /etc/apt/apt.conf.d/50appstream

Can be disabled with:

sudo mv /etc/apt/apt.conf.d/50appstream{,.disabled}

That stopped it on mine, still visible in updates as:

N: Ignoring file '50appstream.disable' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension

Edit:

If you want, you can just move the file outside of that directory, it will no longer show up. No more notices. eg:sudo mv /etc/apt/apt.conf.d/50appstream /etc/apt/50appstream