/usr/share/fish/completions/a2ensite.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 | complete -c a2ensite -s q -l quiet -d "Don't show informative messages"
complete -c a2ensite -xa '(__fish_print_debian_apache_sites)'
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