Since v7.10, Logstash prints the following warning message when this
setting is not defined:

[deprecation.logstash.outputs.elasticsearch] Relying on default value of
`pipeline.ecs_compatibility`, which may change in a future major release
of Logstash. To avoid unexpected changes when upgrading Logstash, please
explicitly declare your desired ECS Compatibility mode.

Ref: https://www.elastic.co/guide/en/logstash/7.10/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-ecs_compatibility