Our AWS ElasticSearch terraform module now supports auto-configuration for multi-az deployment. The criteria is to always enable multiple Availability Zones up to 3 zones and within the available resources, unless specified otherwise by the user.
Your lead engineer will take care of applying the changes to your Elasticsearch cluster(s) when applicable. You will find a Github issue on your board for tracking this.
To know more about what multi-az deployment is and what it means for your Elastisearch domain you can check the AWS documentation here