https://www.elastic.co/guide/en/elasticsearch/reference/current/release-notes-6.3.0.html

The 6.3.0 release notes mention a few features that as far as I can tell didn't actually make it into the release, for example:

Update s3 secure settings #28517
Update ec2 secure settings #29134

It seems these PRs were merged into a feature branch then eventually made their way into 7.0.0 & 6.4.0 via #29135