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.