Is it mandatory to load the index template into Elasticsearch manually when using filebeat?

Elastic Stack Beats
2

It depends on where you are writing the Filebeat data to and whether the index template you have contains the appropriate mappings for the Filebeat fields. To be safe I would follow the directions.

My recommendation is to install the index template provided by Filebeat and write your data into the prescribed filebeat-<version>-* indices. Basically following the documentation and using the defaults. So you would manually load the Filebeat template as per the instructions. This template will apply to indices matching filebeat-6.2.4-*.

Then configure Logstash to output the beat data with this config.

output {
  elasticsearch {
    hosts => "localhost:9200"
    manage_template => false
    index => "%{[@metadata][beat]}-%{[@metadata][version]}-%{+YYYY.MM.dd}" 
    document_type => "%{[@metadata][type]}" 
  }
}

If you make any adjustments to index naming pattern then the template also needs to be changed.