/usr/share/fish/completions/apt-config.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #apt-config
complete -c apt-config -s h -l help --description "Display help and exit"
complete -c apt-config -a shell --description "Access config file from shell"
complete -f -c apt-config -a dump --description "Dump contents of config file"
complete -f -c apt-config -s v -l version --description "Display version and exit"
complete -r -c apt-config -s c -l config-file --description "Specify config file"
complete -x -c apt-config -s o -l option --description "Specify options"
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