Add note explaining absence of docs.

docs/developer/index.asciidoc

@@ -15,6 +15,8 @@ include::core-development.asciidoc[]
 
 include::plugin-development.asciidoc[]
 
+include::visualize/development-visualize-index.asciidoc[]
+
 include::add-data-guide.asciidoc[]
 
 include::security/index.asciidoc[]

docs/developer/visualize/development-visualize-index.asciidoc

@@ -0,0 +1,26 @@
+[[development-visualize-index]]
+== Developing Visualizations
+
+[IMPORTANT]
+==============================================
+These pages document internal APIs and are not guaranteed to be supported across future versions of Kibana.
+==============================================
+
+The internal APIs for creating custom visualizations are in a state of heavy churn as
+they are being migrated to the new Kibana platform, and large refactorings have been
+happening across minor releases in the `7.x` series. In particular, in `7.5` and later
+we have made significant changes to the legacy APIs as we work to gradually replace them.
+
+As a result, starting in `7.5` we have removed the documentation for the legacy APIs
+to prevent confusion. We expect to be able to create new documentation later in `7.x`
+when the visualizations plugin has been completed.
+
+We would recommend waiting until later in `7.x` to upgrade your plugins if possible.
+If you would like to keep up with progress on the visualizations plugin in the meantime,
+here are a few resources:
+
+* The <<breaking-changes,breaking changes>> documentation, where we try to capture any changes to the APIs as they occur across minors.
+* link:https://github.com/elastic/kibana/issues/44121[Meta issue] which is tracking the move of the plugin to the new Kibana platform
+* Our link:https://www.elastic.co/blog/join-our-elastic-stack-workspace-on-slack[Elastic Stack workspace on Slack].
+* The {repo}blob/{branch}/src/plugins/visualizations[source code], which will continue to be
+the most accurate source of information.