Most of the classes have been marked as @final and they will be final in 4.0.
There is a minimal BC Break on MessageManager::getPager. If you are extending this method you should add parameter and return type hints.
Controllers for NelmioApiDocBundle v2 were moved under Sonata\NotificationBundle\Controller\Api\Legacy\ namespace and controllers for NelmioApiDocBundle v3 were added as replacement. If you extend them, you MUST ensure they are using the corresponding inheritance.
Registering SonataEasyExtendsBundle bundle is deprecated, it SHOULD NOT be registered.
Register SonataDoctrineBundle bundle instead.
You might face some issues (though we tried to keep everything BC) if you are extending AMQPBackend, AMQPBackendDispatcher or AMQPMessageIterator classes
or rely on their __construct method signature.
If you want to migrate to another amqp interop compatible transport, say enqueue/amqp-ext, the BC layer want work and the exception is thrown.
All files under the Tests directory are now correctly handled as internal test classes.
You can't extend them anymore, because they are only loaded when running internal tests.
More information can be found in the composer docs.