/usr/share/fish/completions/chsh.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.
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# Completions for the chsh command
#
# This grep tries to match nonempty lines that do not start with hash
complete -c chsh -s s -l shell -x -a "(string match -r '^[^#].*' < /etc/shells)" -d "Specify your login shell"
complete -c chsh -s u -l help -d "Display help and exit "
complete -c chsh -s v -l version -d "Display version and exit"
complete -x -c chsh -a "(__fish_complete_users)"
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