/usr/share/bash-completion/completions/systemctl is in systemd 229-4ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#
# This file is part of systemd.
#
# Copyright 2010 Ran Benita
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version.
#
# systemd is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with systemd; If not, see <http://www.gnu.org/licenses/>.
__systemctl() {
        local mode=$1; shift 1
        systemctl $mode --full --no-legend "$@"
}
__systemd_properties() {
        local mode=$1
        { __systemctl $mode show --all;
         /lib/systemd/systemd --dump-configuration-items; } |
        while IFS='=' read -r key value; do
            [[ $value ]] && echo "$key"
        done
}
__contains_word () {
        local w word=$1; shift
        for w in "$@"; do
                [[ $w = "$word" ]] && return
        done
}
__filter_units_by_property () {
        local mode=$1 property=$2 value=$3 ; shift 3
        local units=("$@")
        local props
        IFS=$'\n' read -rd '' -a props < \
            <(__systemctl $mode show --property "$property" -- "${units[@]}")
        for ((i=0; $i < ${#units[*]}; i++)); do
                if [[ "${props[i]}" = "$property=$value" ]]; then
                        echo " ${units[i]}"
                fi
        done
}
__get_all_units      () { { __systemctl $1 list-unit-files; __systemctl $1 list-units --all; } \
        | { while read -r a b; do [[ $a =~ @\. ]] || echo " $a"; done; }; }
__get_template_names () { __systemctl $1 list-unit-files \
        | { while read -r a b; do [[ $a =~ @\. ]] && echo " ${a%%@.*}@"; done; }; }
__get_active_units   () { __systemctl $1 list-units       \
        | { while read -r a b; do echo " $a"; done; }; }
__get_startable_units () {
        # find startable inactive units
        __filter_units_by_property $mode ActiveState inactive $(
            __filter_units_by_property $mode CanStart yes $(
                __systemctl $mode list-unit-files --state enabled,disabled,static | \
                    { while read -r a b; do [[ $a =~ @\. ]] || echo " $a"; done; }
                __systemctl $mode list-units --state inactive,failed | \
                    { while read -r a b; do echo " $a"; done; } ))
}
__get_restartable_units () {
        # filter out masked and not-found
        __filter_units_by_property $mode CanStart yes $(
            __systemctl $mode list-unit-files --state enabled,disabled,static | \
                    { while read -r a b; do [[ $a =~ @\. ]] || echo " $a"; done; }
            __systemctl $mode list-units | \
                    { while read -r a b; do echo " $a"; done; } )
}
__get_failed_units   () { __systemctl $1 list-units       \
        | { while read -r a b c d; do [[ $c == "failed"   ]] && echo " $a"; done; }; }
__get_enabled_units  () { __systemctl $1 list-unit-files  \
        | { while read -r a b c  ; do [[ $b == "enabled"  ]] && echo " $a"; done; }; }
__get_disabled_units () { __systemctl $1 list-unit-files  \
        | { while read -r a b c  ; do [[ $b == "disabled" ]] && echo " $a"; done; }; }
__get_masked_units   () { __systemctl $1 list-unit-files  \
        | { while read -r a b c  ; do [[ $b == "masked"   ]] && echo " $a"; done; }; }
__get_all_unit_files () { { __systemctl $1 list-unit-files; } | { while read -r a b; do echo " $a"; done; }; }
__get_machines() {
        local a b
        { machinectl list-images --no-legend --no-pager; machinectl list --no-legend --no-pager; } | \
		{ while read a b; do echo " $a"; done; }
}
_systemctl () {
        local cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
        local i verb comps mode
        local -A OPTS=(
               [STANDALONE]='--all -a --reverse --after --before --defaults --failed --force -f --full -l --global
                             --help -h --no-ask-password --no-block --no-legend --no-pager --no-reload --no-wall
                             --quiet -q --privileged -P --system --user --version --runtime --recursive -r --firmware-setup
                             --show-types -i --ignore-inhibitors --plain'
                      [ARG]='--host -H --kill-who --property -p --signal -s --type -t --state --job-mode --root
                             --preset-mode -n --lines -o --output -M --machine'
        )
        if __contains_word "--user" ${COMP_WORDS[*]}; then
            mode=--user
        elif __contains_word "--global" ${COMP_WORDS[*]}; then
            mode=--user
        else
            mode=--system
        fi
        if __contains_word "$prev" ${OPTS[ARG]}; then
                case $prev in
                        --signal|-s)
                                _signals
                                return
                        ;;
                        --type|-t)
                                comps=$(__systemctl $mode -t help)
                        ;;
                        --state)
                                comps=$(__systemctl $mode --state=help)
                        ;;
                        --job-mode)
                                comps='fail replace replace-irreversibly isolate
                                       ignore-dependencies ignore-requirements flush'
                        ;;
                        --kill-who)
                                comps='all control main'
                        ;;
                        --root)
                                comps=$(compgen -A directory -- "$cur" )
                                compopt -o filenames
                        ;;
                        --host|-H)
                                comps=$(compgen -A hostname)
                        ;;
                        --property|-p)
                                comps=$(__systemd_properties $mode)
                        ;;
                        --preset-mode)
                                comps='full enable-only disable-only'
                        ;;
                        --output|-o)
                                comps='short short-iso short-precise short-monotonic verbose export json
                                       json-pretty json-sse cat'
                        ;;
                        --machine|-M)
                                comps=$( __get_machines )
                        ;;
                esac
                COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
                return 0
        fi
        if [[ "$cur" = -* ]]; then
                COMPREPLY=( $(compgen -W '${OPTS[*]}' -- "$cur") )
                return 0
        fi
        local -A VERBS=(
                [ALL_UNITS]='is-active is-failed is-enabled status show cat mask preset help list-dependencies edit set-property'
            [ENABLED_UNITS]='disable'
           [DISABLED_UNITS]='enable'
        [REENABLABLE_UNITS]='reenable'
             [FAILED_UNITS]='reset-failed'
          [STARTABLE_UNITS]='start'
          [STOPPABLE_UNITS]='stop condstop kill try-restart condrestart'
         [ISOLATABLE_UNITS]='isolate'
         [RELOADABLE_UNITS]='reload condreload try-reload-or-restart force-reload'
        [RESTARTABLE_UNITS]='restart reload-or-restart'
         [TARGET_AND_UNITS]='add-wants add-requires'
             [MASKED_UNITS]='unmask'
                     [JOBS]='cancel'
                     [ENVS]='set-environment unset-environment'
               [STANDALONE]='daemon-reexec daemon-reload default
                             emergency exit halt hibernate hybrid-sleep kexec list-jobs
                             list-sockets list-timers list-units list-unit-files poweroff
                             reboot rescue show-environment suspend get-default
                             is-system-running'
                     [FILE]='link switch-root'
                  [TARGETS]='set-default'
        )
        for ((i=0; i < COMP_CWORD; i++)); do
                if __contains_word "${COMP_WORDS[i]}" ${VERBS[*]} &&
                 ! __contains_word "${COMP_WORDS[i-1]}" ${OPTS[ARG]}; then
                        verb=${COMP_WORDS[i]}
                        break
                fi
        done
        if [[ -z $verb ]]; then
                comps="${VERBS[*]}"
        elif __contains_word "$verb" ${VERBS[ALL_UNITS]}; then
                comps=$( __get_all_units $mode )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[ENABLED_UNITS]}; then
                comps=$( __get_enabled_units $mode )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[DISABLED_UNITS]}; then
                comps=$( __get_disabled_units $mode;
                        __get_template_names $mode)
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[REENABLABLE_UNITS]}; then
                comps=$( __get_disabled_units $mode;
                         __get_enabled_units $mode;
                         __get_template_names $mode)
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[STARTABLE_UNITS]}; then
                comps=$( __get_startable_units $mode;
                         __get_template_names $mode)
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[RESTARTABLE_UNITS]}; then
                comps=$( __get_restartable_units $mode;
                         __get_template_names $mode)
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[STOPPABLE_UNITS]}; then
                comps=$( __filter_units_by_property $mode CanStop yes \
                      $( __get_active_units $mode ) )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[RELOADABLE_UNITS]}; then
                comps=$( __filter_units_by_property $mode CanReload yes \
                      $( __get_active_units $mode ) )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[ISOLATABLE_UNITS]}; then
                comps=$( __filter_units_by_property $mode AllowIsolate yes \
                      $( __get_all_units $mode ) )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[FAILED_UNITS]}; then
                comps=$( __get_failed_units $mode )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[MASKED_UNITS]}; then
                comps=$( __get_masked_units $mode )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[TARGET_AND_UNITS]}; then
                if __contains_word "$prev" ${VERBS[TARGET_AND_UNITS]} \
                || __contains_word "$prev" ${OPTS[STANDALONE]}; then
                        comps=$( __systemctl $mode list-unit-files --type target --all \
                        | { while read -r a b; do echo " $a"; done; } )
                else
                        comps=$( __get_all_unit_files $mode )
                fi
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[STANDALONE]}; then
                comps=''
        elif __contains_word "$verb" ${VERBS[JOBS]}; then
                comps=$( __systemctl $mode list-jobs | { while read -r a b; do echo " $a"; done; } )
        elif __contains_word "$verb" ${VERBS[ENVS]}; then
                comps=$( __systemctl $mode show-environment \
                    | while read -r line; do echo " ${line%%=*}=";done )
                compopt -o nospace
        elif __contains_word "$verb" ${VERBS[FILE]}; then
                comps=$( compgen -A file -- "$cur" )
                compopt -o filenames
        elif __contains_word "$verb" ${VERBS[TARGETS]}; then
                comps=$( __systemctl $mode list-unit-files --type target --full --all \
                        | { while read -r a b; do echo " $a"; done; } )
        fi
        COMPREPLY=( $(compgen -o filenames -W '$comps' -- "$cur") )
        return 0
}
complete -F _systemctl systemctl
 |