Actually this is a bug in the migration guide (which I already removed). And we also need to remove it from XSD.
Here is what's going on. Both Aggregator and Resequencer underwent major refactoring for 2.1 and one fo the major change was that we no longer keep released messages as we did in previous versions since the only place where we did keep messages was in Resequencer and only to know what was the last released message. Right now we simply remember the sequence number of the last released message via 'lastReleasedSequenceNumber'
Basically we've introduced a new attribute for backwards compatibility but after few discussions realized that we really don't need that but never cleaned up the schema.