/usr/share/fish/completions/dpkg.fish is in fish-common 2.2.0-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | complete -c dpkg -s i -l install -d 'Install .deb package' -xa '(__fish_complete_suffix .deb)'
complete -c dpkg -s r -l remove -d 'Remove package' -xa '(__fish_print_packages)'
complete -c dpkg -s P -l purge -d 'Purge package' -xa '(__fish_print_packages)'
complete -c dpkg -l force-all -d 'Continue on all problems'
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