I have used the script below to suppress the icloud and diagnostic popup for new and existing users from 10.10 to 10.12, but now with 10.13 it is not working to suppress the icloud popup for existing users. It will suppress the diagnostics for existing users and new users.

here is that script. i don't see what could have changed that would allow existing users to get the icloud login screen.

#!/bin/bash

# Initial setup script for Mac OS X 10.10.x
# https://github.com/rtrouton/rtrouton_scripts/blob/master/rtrouton_scripts/first_boot/10.10/first_boot.sh

# Determine OS version and build version
# as part of the following actions to disable
# the iCloud and Diagnostic pop-up windows

osvers=$(sw_vers -productVersion | awk -F. '{print $2}')
sw_vers=$(sw_vers -productVersion)
sw_build=$(sw_vers -buildVersion)


# Checks first to see if the Mac is running 10.7.0 or higher. 
# If so, the script checks the system default user template
# for the presence of the Library/Preferences directory.
#
# If the directory is not found, it is created and then the
# iCloud and Diagnostic pop-up settings are set to be disabled.

if [[ ${osvers} -ge 7 ]]; then

 for USER_TEMPLATE in "/System/Library/User Template"/*
  do
    /usr/bin/defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant DidSeeCloudSetup -bool TRUE
    /usr/bin/defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant GestureMovieSeen none
    /usr/bin/defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant LastSeenCloudProductVersion "${sw_vers}"
    /usr/bin/defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant LastSeenBuddyBuildVersion "${sw_build}"
    /usr/bin/defaults write "${USER_TEMPLATE}"/Library/Preferences/com.apple.SetupAssistant DidSeeSiriSetup -bool TRUE
  done

 # Checks first to see if the Mac is running 10.7.0 or higher.
 # If so, the script checks the existing user folders in /Users
 # for the presence of the Library/Preferences directory.
 #
 # If the directory is not found, it is created and then the
 # iCloud and Diagnostic pop-up settings are set to be disabled.

 for USER_HOME in /Users/*
  do
    USER_UID=`basename "${USER_HOME}"`
    if [ ! "${USER_UID}" = "Shared" ] 
    then 
      if [ ! -d "${USER_HOME}"/Library/Preferences ]
      then
        mkdir -p "${USER_HOME}"/Library/Preferences
        chown "${USER_UID}" "${USER_HOME}"/Library
        chown "${USER_UID}" "${USER_HOME}"/Library/Preferences
      fi
      if [ -d "${USER_HOME}"/Library/Preferences ]
      then
        /usr/bin/defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant DidSeeCloudSetup -bool TRUE
        /usr/bin/defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant GestureMovieSeen none
        /usr/bin/defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant LastSeenCloudProductVersion "${sw_vers}"
        /usr/bin/defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant LastSeenBuddyBuildVersion "${sw_build}"
        /usr/bin/defaults write "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant DidSeeSiriSetup -bool TRUE
        chown "${USER_UID}" "${USER_HOME}"/Library/Preferences/com.apple.SetupAssistant.plist
      fi
    fi
  done
fi

# Set whether you want to send diagnostic info back to
# Apple and/or third party app developers. If you want
# to send diagonostic data to Apple, set the following 
# value for the SUBMIT_DIAGNOSTIC_DATA_TO_APPLE value:
#
# SUBMIT_DIAGNOSTIC_DATA_TO_APPLE=TRUE
# 
# If you want to send data to third party app developers,
# set the following value for the
# SUBMIT_DIAGNOSTIC_DATA_TO_APP_DEVELOPERS value:
#
# SUBMIT_DIAGNOSTIC_DATA_TO_APP_DEVELOPERS=TRUE
# 
# By default, the values in this script are set to 
# send no diagnostic data: 

SUBMIT_DIAGNOSTIC_DATA_TO_APPLE=FALSE
SUBMIT_DIAGNOSTIC_DATA_TO_APP_DEVELOPERS=FALSE

# To change this in your own script, comment out the FALSE
# lines and uncomment the TRUE lines as appropriate.

# Checks first to see if the Mac is running 10.10.0 or higher. 
# If so, the desired diagnostic submission settings are applied.

if [[ ${osvers} -ge 10 ]]; then

  CRASHREPORTER_SUPPORT="/Library/Application Support/CrashReporter"

  if [ ! -d "${CRASHREPORTER_SUPPORT}" ]; then
    mkdir "${CRASHREPORTER_SUPPORT}"
    chmod 775 "${CRASHREPORTER_SUPPORT}"
    chown root:admin "${CRASHREPORTER_SUPPORT}"
  fi

 /usr/bin/defaults write "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory AutoSubmit -boolean ${SUBMIT_DIAGNOSTIC_DATA_TO_APPLE}
 /usr/bin/defaults write "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory AutoSubmitVersion -int 4
 /usr/bin/defaults write "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory ThirdPartyDataSubmit -boolean ${SUBMIT_DIAGNOSTIC_DATA_TO_APP_DEVELOPERS}
 /usr/bin/defaults write "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory ThirdPartyDataSubmitVersion -int 4
 /bin/chmod a+r "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory.plist
 /usr/sbin/chown root:admin "$CRASHREPORTER_SUPPORT"/DiagnosticMessagesHistory.plist

fi

exit 0