For new versions of HCA we will proactively report state of endpoints to meet the v3 API requirements, however, for existing and older versions the implementation will be based on the end point state being retrievable.
We have implemented this and noticed that while the state cannot be retrieved via voice command, it can be retrieved via the new version of the Alexa App. This is however very resource intensive as each device is a separate call to the Lambda function. I imagine this will rapidly use up the free Lamda daily quota, in addition it is adding considerable load to our cloud database as each Lamda call requires queries and updates.
When ReportState is needed for multiple end points, this should be made as a single Lambda request rather than separate.