From the 8.4.0.0 version of Sentinel, the in-built Elasticsearch is bundled with an X-Pack security plug-in, out of the box. X-Pack plugin is an Elastic Stack extension that provides security and other capabilities. With this change, Sentinel now only communicates with Elasticsearch (which is used to store alerts and event data) over SSL. Also, all the Elasticsearch node-to-node cluster communications too are now mandated over SSL, thereby making the data transfer across Elasticsearch nodes more secure. This enhanced security setting is now mandated from Sentinel 8.4.0.0 onwards and the users are expected to complete few manual steps like certificate configuration, as part of their install/upgrade process.
For more information about securing communication with Elasticsearch, see the Configuring Elasticsearch for Event Visualization section.