I succeed to use a external ElasticSearch with latest sonarqube 8.9. But it's just a hack at your own risk.
Steps
Create a elastic search server
First start a elastic search instance anywhere.
Modify the config files
Modify the file
cat >> conf/sonar.properties < EOF
# your external host and port
sonar.search.port=9200
sonar.search.host=192.168.xx.xx
EOF
# create a dummy run script
cat > elasticsearch/bin/elasticsearch << EOF
#!/bin/bash
# it's a inflate sleep
cat
EOF
Run sonarqube
just start sonarqube and view indexs in your new elasticsearch.