Just as the title says: Can a Custom Realm be installed in Elastic Cloud?
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These are the supported Security configurations in Elastic Cloud - Securing your deployment | Elasticsearch Service Documentation | Elastic
See Elastic Cloud Feature Matrix | Elastic
Under Elastic Stack security there is a row for
Custom authentication and authorization realms
Technically a custom realm is a special Elasticsearch plugin, so you should read through the documentation about loading plugins in the Elastic Cloud Elasticsearch Service