[DOCS] Add deprecation docs for 6.x and earlier indices

docs/reference/migration/index.asciidoc

@@ -28,6 +28,7 @@ For more information about {minor-version},
 see the <<release-highlights>> and <<es-release-notes>>.
 For information about how to upgrade your cluster, see <<setup-upgrade>>.
 
+* <<migrating-7.15,Migrating to 7.15>>
 * <<breaking-changes-7.14,Migrating to 7.14>>
 * <<breaking-changes-7.13,Migrating to 7.13>>
 * <<breaking-changes-7.12,Migrating to 7.12>>
@@ -46,6 +47,7 @@ For information about how to upgrade your cluster, see <<setup-upgrade>>.
 
 --
 
+include::migrate_7_15.asciidoc[]
 include::migrate_7_14.asciidoc[]
 include::migrate_7_13.asciidoc[]
 include::migrate_7_12.asciidoc[]

docs/reference/migration/migrate_7_15.asciidoc

@@ -0,0 +1,73 @@
+[[migrating-7.15]]
+== Migrating to 7.15
+++++
+<titleabbrev>7.15</titleabbrev>
+++++
+
+This section discusses the changes that you need to be aware of when migrating
+your application to {es} 7.15.
+
+See also <<release-highlights>> and <<es-release-notes>>.
+
+////
+//NOTE: The notable-breaking-changes tagged regions are re-used in the
+//Installation and Upgrade Guide
+
+[discrete]
+[[breaking-changes-7.15]]
+=== Breaking changes
+
+The following changes in {es} 7.15 might affect your applications
+and prevent them from operating normally.
+Before upgrading to 7.15, review these changes and take the described steps
+to mitigate the impact.
+
+NOTE: Breaking changes introduced in minor versions are
+normally limited to security and bug fixes.
+Significant changes in behavior are deprecated in a minor release and
+the old behavior is supported until the next major release.
+To find out if you are using any deprecated functionality,
+enable <<deprecation-logging, deprecation logging>>.
+
+// tag::notable-breaking-changes[]
+// end::notable-breaking-changes[]
+////
+
+[discrete]
+[[deprecated-7.15]]
+=== Deprecations
+
+The following functionality has been deprecated in {es} 7.15 and will be removed
+in 8.0. While this won't have an immediate impact on your applications, we
+strongly encourage you take the described steps to update your code after
+upgrading to 7.15.
+
+NOTE: Significant changes in behavior are deprecated in a minor release and the
+old behavior is supported until the next major release. To find out if you are
+using any deprecated functionality, enable <<deprecation-logging, deprecation
+logging>>.
+
+// tag::notable-breaking-changes[]
+[[deprecate-6x-indices]]
+.Indices created in Elasticsearch 6.x and earlier versions are deprecated.
+[%collapsible]
+====
+*Details* +
+In 8.x, {es} will only read indices created in version 7.0 or above. An 8.x node
+will not start in the presence of indices created in 6.x or earlier versions of
+{es}.
+
+*Impact* +
+Before upgrading to an 8.x version, reindex any index or data stream created in
+6.x or earlier versions with {es} 7.x. If you no longer need the index or data
+stream, delete it instead.
+
+You can use the get index API to check the {es} version in which an index
+was created.
+
+[source,console]
+----
+GET *,-.*?human=true&filter_path=**.settings.index.version.created_string
+----
+====
+// end::notable-breaking-changes[]