Call it with sudo bash Disable_macOS_upgrade.sh disable Catalina

Disable_macOS_upgrade.sh

#!/bin/bash
# Call it with eg. sudo bash Disable_macOS_upgrade.sh disable Catalina

disable_osxnotification() {
if [ -d /Library/Bundles/OSXNotification.bundle ]; then
	hdiutil create -format UDZO -fs hfs+ -srcfolder /Library/Bundles/OSXNotification.bundle /Library/Bundles/OSXNotification
	rm -rf /Library/Bundles/OSXNotification.bundle
fi
if [[ ! -e /Library/Bundles/OSXNotification.bundle ]]; then
	touch /Library/Bundles/OSXNotification.bundle
	chflags schg /Library/Bundles/OSXNotification.bundle
fi
}

disable_release() {
NAMEOFRELEASE=$1
if [ -d "/Applications/Install macOS ${NAMEOFRELEASE}.app" ]; then
	rm -rf "/Applications/Install macOS ${NAMEOFRELEASE}.app"
fi
if [[ ! -e "/Applications/Install macOS ${NAMEOFRELEASE}.app" ]]; then
	touch "/Applications/Install macOS ${NAMEOFRELEASE}.app"
	chflags hidden "/Applications/Install macOS ${NAMEOFRELEASE}.app"
	chflags schg  "/Applications/Install macOS ${NAMEOFRELEASE}.app"
	touch "/Applications/macOS ${NAMEOFRELEASE}.appdownload"
	chflags hidden "/Applications/macOS ${NAMEOFRELEASE}.appdownload"
	chflags schg "/Applications/macOS ${NAMEOFRELEASE}.appdownload"
	
fi
}

re-enable_osxnotification() {
	if [[ -f /Library/Bundles/OSXNotification.bundle && -f /Library/Bundles/OSXNotification.dmg ]]; then
		chflags noschg /Library/Bundles/OSXNotification.bundle
		rm /Library/Bundles/OSXNotification.bundle
		MP=$(diskutil info $(hdiutil attach -private /Library/Bundles/OSXNotification.dmg | tail -n1 |awk '{print $1}')|grep Mount\ Point|sed -E 's/Mount Point://'|sed -E 's/^[^/]+//')
		ditto "$MP/OSXNotification.bundle/" /Library/Bundles/OSXNotification.bundle
		hdiutil detach "$MP"
		rm /Library/Bundles/OSXNotification.dmg
	elif [ -f /Library/Bundles/OSXNotification.bundle ]; then
		chflags noschg /Library/Bundles/OSXNotification.bundle
		rm /Library/Bundles/OSXNotification.bundle
	fi	
}

remove_release_block() {
	NAMEOFRELEASE=$1
	if [ -f "/Applications/Install macOS ${NAMEOFRELEASE}.app" ]; then
		chflags noschg "/Applications/Install macOS ${NAMEOFRELEASE}.app"
		rm "/Applications/Install macOS ${NAMEOFRELEASE}.app"
	fi
	if [ -f "/Applications/macOS ${NAMEOFRELEASE}.appdownload" ]; then
		chflags noschg "/Applications/macOS ${NAMEOFRELEASE}.appdownload"
		rm "/Applications/macOS ${NAMEOFRELEASE}.appdownload"
	fi
}

OP=$1
RELEASE=$2
if [ "$OP" = "disable" ]; then
  disable_osxnotification
  disable_release "$RELEASE"
elif [ "$OP" = "reenable" ]; then
  remove_release_block "$RELEASE"
  re-enable_osxnotification
fi

exit 0