4.3.0 release notes update: mention #14769

even though that PR hasn't been merge yet, it will
be and for now, it is the biggest 4.3-specific change,
so let's list it.

Author: michaelklishin

release-notes/4.3.0.md

@@ -1,9 +1,23 @@
 ## RabbitMQ 4.3.0
 
+RabbitMQ `4.3.0` is a new feature release.
+
 
 ## Breaking Changes and Compatibility Notes
 
-TBD
+### Classic Queues v1 Storage (CQv1) is Removed
+
+This release removes the original classic queue storage implementation these days
+known as CQv1. A 2nd generation implementation called CQv2 has been adopted
+as the default starting with `4.2.0`.
+
+This means that attemptes to declare a queue using the following [optional queue arguments](https://www.rabbitmq.com/docs/queues#optional-arguments) will fail:
+
+ * `x-queue-mode` set to any value
+ * `x-queue-version` set to `1`
+
+Existing classic queues upgraded to CQv2 during an earlier upgrade to `4.2.x` will continue
+operating as usual.
 
 
 ## Release Highlights
@@ -18,7 +32,8 @@ TBD
 See the [Upgrading guide](https://www.rabbitmq.com/docs/upgrade) for documentation on upgrades and [GitHub releases](https://github.com/rabbitmq/rabbitmq-server/releases)
 for release notes of individual releases.
 
-This release series supports upgrades from `4.2.x`.
+This release series supports upgrades from `4.2.x`. Upgrades from earlier series are **not** supported:
+users must upgrade to the latest available `4.2.x` patch release before upgrading to `4.3.0`.
 
 
 ### New Required Feature Flags