Add AMQP 0.9.1 x-death header to breaking changes

Starting with RabbitMQ 4.0, the AMQP 0.9.1 x-death header won't be
interpreted anymore by RabbitMQ when clients publish new messages to RabbitMQ.

Relates to
* #10709
* #11331
* #11339

Author: ansd

release-notes/4.0.1.md

@@ -67,6 +67,17 @@ the delivery limit can be disabled by setting a delivery limit configuration of
 strongly recommends keeping the delivery limit in place to ensure cluster availability isn't
 accidentally sacrificed.
 
+### AMQP 0.9.1 x-death header
+
+Up to RabbitMQ 3.13, when an AMQP 0.9.1 client (re-)published a message to RabbitMQ, RabbitMQ interpreted the
+AMQP 0.9.1 [x-death](https://www.rabbitmq.com/docs/dlx#effects) header in the published message's `basic_message.content.properties.headers` field.
+
+RabbitMQ 4.x will not interpret this `x-death` header anymore when clients (re-)publish a message.
+Note that RabbitMQ 4.x will continue to set and update the `x-death` header every time a message is dead-lettered, including when a client **rejects** the message.
+
+If you have a use case where you relied on RabbitMQ incrementing the `count` fields within the `x-death` header array elements for new messages **(re-)published**
+(instead of existing messages being rejected), consider introducing and incrementing [your own custom non `x-` header](https://github.com/rabbitmq/rabbitmq-server/issues/10709#issuecomment-1997083246) instead.
+
 ### CQv1 Storage Implementation was Removed
 
 CQv1, [the original classic queue storage layer, was removed](https://github.com/rabbitmq/rabbitmq-server/pull/10656)