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What's DEP-11 index files for?
It's a part of AppStream technology implemented on Debian GNU/Linux and family distros today so the "Software Center" program could display software info which are user-friendly and easy to install. In a simplest sense for Ubuntu users, the DEP-11 is specific index file format for Ubuntu Software. So whenever you run apt-get update command, it will always download the regular files + DEP-11 files redundantly.
How to disable it?
Delete the 50appstream file on /etc/apt/apt.conf.d/ directory.
sudo rm /etc/apt/apt.conf.d/50appstream
Or if you wish, backup it first and than delete the actual file.
sudo mv /etc/apt/apt.conf.d/50appstream /etc/apt/50appstream_backup
sudo rm /etc/apt/apt.conf.d/50appstream
This will disable downloading DEP-11 permanently.
What's the consequence?
The apt-get update command will no longer download any of DEP-11 index files from anywhere. This results in bandwidth saving. However, you wouldn't be able to use Ubuntu Software properly (and this should be OK if you prefer to use command lines).
Reference
https://askubuntu.com/questions/823329/how-do-i-disable-fetching-of-dep-11-files