Xenial (16.04) is on a development branch until final release, having maintained all updates means you are already up-to-date! "Beta 2" is just like a label and snapshot, not an upgrade. 
I remember you were looking to keep "Beta 1" exactly the way it was. The only way to do that is to not update (too late now!)
Proposed updates are only for testing updates and providing development feedback. Enabling this may introduce instability.
Enabling this probably isn't a good idea unless you were testing something for a developer.