I suspected this was going to happen, but was just made aware of the official statement (thanks Mike and Christian for highlighting the info)
From the 12c "Database Upgrade Guide" at https://docs.oracle.com/database/121/UPGRD/deprecated.htm#BABDBCJI
"The non-CDB architecture is deprecated in Oracle Database 12c, and may be desupported and unavailable in a later Oracle Database release. Oracle recommends use of the CDB architecture."
(There is also a note about features that are not ready in CDB architecture at the same location.)
I especially like Mike Dietrich's discussion at https://blogs.oracle.com/UPGRADE/entry/non_cdb_architecture_of_oracle
/Hans
