As always, there has to be someone pointing out the issue or solution. In this case it was Mike Dietrich (Mr. Upgrade). In the Oracle Upgrade Guide for 12c it is in black and white:
Cite: “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.” (Oracle 12c Database Upgrade Guide, chapter 8.1.1).
But the multitenant database option is charged. Does this mean you have to buy this option first to migrate to Oracle 12c?
Definitely not, because the so called „single tenant“ function i.e. a CDB with only one PDB (beside the obligatory PDB$SEED) can be used without an additional license:
Cite: “The multitenant architecture with one pluggable database (single tenant) is available in all editions.” (Oracle 12c Database Licensing Information Table 1.1)
Find further information in Mike Dietrich’s blog.