In our project we have a GOLD elastic license, but in order to put it into elastic cluster it says that we need to enable TLS or disable security (setting xpack.security.enabled: false).
TLS for the transport layer is a requirement after 6.0 for enabling X-Pack security. However, if for some reason you want your clients to communicate and authenticate to your cluster over plaintext HTTP, you can keep TLS on the http layer disabled (although this is not considered best practice)
See this for enabling TLS on transport layer only.
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