The 12c documentation says that Oracle Restart is depreciated and may be de-supported in a future release...
Does anyone have any insight into this decision?
If you look at the DBA doc section on 'Oracle Restart' you will find your likely reason:
Oracle Restart is used in standalone server (non-clustered) environments only. For Oracle Real Application Clusters (Oracle RAC) environments, the functionality to automatically restart components is provided by Oracle Clusterware.
Now that Oracle 12c is released and implements 'pluggable databases' functionality like 'Oracle Restart' will likely be handled by the superstructure code of 12c. That is, there will be no need for such 'standalone server' functionality since the pluggable functionality will be responsible for that.