Q. With Nano Server being CBB does that mean only the two most recent branches will be supported?
A. Yes. While Server with Desktop Experience and Server Core leverage the standard Long Term Servicing Branch (LTSB) model which means versions are supported for the standard 5+5 year support model, Nano Server is Current Branch for Business (CBB) which means new builds will be release frequently and only the two most current builds (plus a small grace period) will be supported and therefore have patches supplied. This means when adopting Nano Server it is critical to have the right processes in place to update the deployments with new builds as required. It is unlikely you would want to "upgrade" instances of Nano Server and would instead simply redeploy the workload on a newer build. Remember Nano Server instances are tin soldiers which we simply replace rather than trying to fix/maintain.