When adding the deprecated property to Settings, we get automatic deprecation logging when the setting is used. The message states:

[xxx] setting was deprecated in Elasticsearch and it will be removed in a future release!
See the breaking changes lists in the documentation for details.

However, the breaking changes are not added until the feature is actually removed in the next major release, so a user seeing this message in 5.3 would have nowhere to see information about what should be used in place of the deprecated setting in that release, since 6.0 is not even released yet. Perhaps we could have a deprecation page per minor release?