/etc/bash_completion.d/chsh is in bash-completion 1:1.3-1ubuntu8.
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 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | # chsh(1) completion
_chsh()
{
    local cur prev
    COMPREPLY=()
    _get_comp_words_by_ref cur prev
    case $prev in
        --list-shells|--help|-v|--version)
            return 0
            ;;
        -s|--shell)
            _shells
            return 0
            ;;
    esac
    if [[ "$cur" == -* && "$( uname -s )" == @(Linux|GNU|GNU/*) ]]; then
        COMPREPLY=( $( compgen -W '--shell --list-shells --help --version' \
            -- "$cur" ) )
    else
        COMPREPLY=( $( compgen -u -- "$cur" ) )
    fi
    return 0
}
complete -F _chsh chsh
# Local variables:
# mode: shell-script
# sh-basic-offset: 4
# sh-indent-comment: t
# indent-tabs-mode: nil
# End:
# ex: ts=4 sw=4 et filetype=sh
 |